Giant Magellan Telescope Scientific Promise and Opportunities

2012 Table of Contents

1 INTRODUCTION ...... 1

1.1 PURPOSE AND SCOPE OF THE DOCUMENT ...... 1 1.2 OVERVIEW OF THE GIANT MAGELLAN TELESCOPE PROJECT ...... 1 1.3 STRUCTURE OF THE GMT SCIENCE CASE ...... 3 1.4 DISCOVERY SPACE OPENED BY THE GMT ...... 4 1.5 CONTEMPORARY SCIENCE GOALS ...... 6 1.6 SCIENTIFIC SYNERGIES ...... 7 1.7 GMT INSTRUMENT CANDIDATES ...... 7 1.8 ADAPTIVE OPTICS SYSTEM ...... 9 1.9 SUMMARY ...... 10 1.10 LIST OF CONTRIBUTORS ...... 11 2 FORMATION OF STARS AND PLANETARY SYSTEMS...... 12

2.1 INTRODUCTION ...... 12 2.1.1 Synergy with other instruments ...... 13 2.2 ASTROPHYSICS OF YOUNG STARS ...... 13 2.3 THE INITIAL MASS FUNCTION AND THE POTENTIAL FOR PLANETS ...... 16 2.4 HOW CIRCUMSTELLAR DISKS FORM PLANETS ...... 17 2.5 VOLATILE DELIVERY AND PLANETARY SYSTEM ARCHITECTURE ...... 20 2.6 SOLAR SYSTEM CLUES TO PLANET FORMATION ...... 22 2.7 SUMMARY ...... 23 REFERENCES ...... 24 3 THE PROPERTIES OF EXOPLANETARY SYSTEMS ...... 25

3.1 INTRODUCTION ...... 25 3.1.1 Current models of Planet Formation ...... 26 3.2 PROBING EXOPLANET ATMOSPHERES ...... 27 3.3 IMAGING EXOPLANETARY SYSTEMS ...... 28 3.3.1 Young Gas‐Giant Planets ...... 30 3.3.2 Spectroscopy in the Near‐Infrared ...... 31 3.3.3 Thermal Imaging of Older and Smaller Planets ...... 31 3.4 PROBING THE NEAREST HABITABLE PLANETS USING DOPPLER SPECTROSCOPY ...... 33 3.4.1 Earth‐analogues Orbiting Sun‐type Stars ...... 33 3.4.2 Habitable planets orbiting M‐dwarf Stars ...... 35 3.5 SUMMARY ...... 36 REFERENCES ...... 37 4 STELLAR POPULATIONS AND CHEMICAL EVOLUTION ...... 38

4.1 INTRODUCTION ...... 38 4.2 STELLAR ARCHAEOLOGY ...... 40 4.2.1 Characterizing the Most Metal‐poor Stars ...... 40 4.2.2 Age Dating the Oldest Stars ...... 42 4.3 ABUNDANCES IN DWARF GALAXY STARS ...... 43 4.4 PROBING THE ORIGIN OF THE MILKY WAY’S HALO ...... 45 4.5 GLOBULAR CLUSTERS IN LOCAL GROUP GALAXIES AND BEYOND ...... 46 4.6 STAR FORMATION IN THE MILKY WAY'S 5KPC RING ...... 47 4.7 SUMMARY ...... 49 REFERENCES ...... 49

5 GALAXY ASSEMBLY AND EVOLUTION ...... 50

5.1 INTRODUCTION ...... 50 5.2 NEAR‐FIELD STUDIES OF GALAXY ASSEMBLY ...... 50 5.3 THE GALAXY BUILDING EPOCH ...... 52 5.3.1 Star Formation, Mass Assembly and Chemical Evolution ...... 52 5.3.2 Dynamical Masses ...... 53 5.3.3 Kinematics of Star Forming Galaxies ...... 55 5.3.4 The Mass‐Metallicity Relation ...... 57 5.4 FEEDBACK AND THE GALAXY‐IGM CONNECTION ...... 58 5.5 THE GALAXY‐BLACK HOLE CONNECTION ...... 61 5.6 SUMMARY ...... 64 REFERENCES ...... 64 6 DARK MATTER, DARK ENERGY AND FUNDAMENTAL PHYSICS ...... 66

6.1 INTRODUCTION ...... 66 6.2 SYNERGY WITH LSST ...... 68 6.3 CLUSTERS AND DARK MATTER ...... 70 6.4 DARK MATTER PROFILES IN DWARF GALAXIES ...... 73 6.5 SUMMARY ...... 75 REFERENCES ...... 75 7 FIRST LIGHT AND REIONIZATION ...... 77

7.1 INTRODUCTION ...... 77 7.2 THE FIRST DARK MATTER HALOS, STARS AND GALAXIES ...... 77 7.3 GALAXIES IN THE EARLY UNIVERSE ...... 79 7.4 DISCOVERING THE FIRST STARS ...... 82 7.5 PROBING REIONIZATION ...... 84 7.5.1 Constraints from Ly Emission ...... 84 7.6 SYNERGY WITH 21CM TOMOGRAPHY ...... 86 7.7 HIGH RESOLUTION SPECTROSCOPY OF QUASARS ...... 87 7.8 SUMMARY ...... 89 REFERENCES ...... 89 8 TRANSIENT PHENOMENA ...... 91

8.1 INTRODUCTION ...... 91 8.2 OPTICAL TRANSIENTS OF LONG‐DURATION GAMMA‐RAY BURSTS ...... 91 8.2.1 The First Stars ...... 95 8.2.2 The Reionization Epoch ...... 96 8.3 SUPERNOVAE ...... 98 8.3.1 Type Ia Supernovae ...... 98 8.3.2 Core‐Collapse Supernovae ...... 100 8.3.3 Super‐Luminous Supernovae ...... 101 8.3.4 Supernova Rates ...... 102 8.4 OTHER TRANSIENT SOURCES ...... 103 8.5 SUMMARY ...... 104 REFERENCES ...... 104 9 SCIENTIFIC SYNERGIES WITH OTHER MAJOR FACILITIES ...... 107

9.1 INTRODUCTION ...... 107 9.2 SYNERGY WITH GROUND‐BASED FACILITIES ...... 108 9.2.1 ALMA ...... 108 9.3 STAR AND PLANET FORMATION ...... 110 9.3.1 The Sub‐Stellar IMF ...... 110 9.3.2 Debris Disks and Protoplanets ...... 111 9.3.3 Star Formation ...... 111 9.4 GMT AND THE LOW FREQUENCY RADIO ARRAYS ...... 112 9.5 LSST ...... 112 9.5.1 Overview ...... 112 9.5.2 Stellar Populations ...... 114 9.5.3 Transients ...... 115 9.5.4 Galaxies and the Early Universe ...... 117 9.6 SOUTHERN SKY SURVEYS ...... 119 9.7 SYNERGY WITH SPACE BASED MISSIONS ...... 120 9.7.1 James Webb Space Telescope ...... 120 9.7.2 Euclid, WFIRST and Other Space‐based Infrared Missions ...... 121 9.7.3 High‐Energy Space Missions ...... 122 9.7.4 Space‐Based Exoplanet Missions ...... 122 9.8 SUMMARY ...... 124 10 GMT SCIENCE CASE SUMMARY ...... 125 11 ABBREVIATIONS AND ACRONYMS ...... 126

1 Introduction Patrick McCarthy

1.1 Purpose and Scope of the Document

This document lays out the scientific motivation for the Giant Magellan Telescope (GMT), its potential scientific instruments, and the critical subsystems that are needed to create a front-line astronomical observatory. As first light for the telescope presently lies some 8-10 years in the future, this document must be forward-looking. We make no attempt to be all encompassing in our discussion of scientific opportunities offered by the GMT; to do so would require a document several times larger than the present volume and would surely remain incomplete in hindsight. Rather, we have attempted to draw a compelling picture of how the GMT will impact a broad spectrum of forefront questions in contemporary astronomy and astrophysics while pointing out the abundant opportunities for new discoveries that the facility will enable.

This document serves multiple purposes; it presents a scientific vision to our professional colleagues and to non-astronomers engaged with large-scale science; it provides a starting point from which the critical requirements for the telescope and subsystems are derived, and it provides a framework against which the impact of technical decisions that have scientific impact can be gauged.

The GMT Science Book has a central place in the GMT document family and flow-down process. The GMT Science Requirements Document draws on the science case presented in this document and sets the detailed flow-down to the GMT System Level Requirements which, in turn, flow down to the GMT Subsystem Level Requirements. Changes in these requirement documents that may have consequences for the scientific capabilities of the facility must be traced up to this document, the Science Book.

1.2 Overview of the Giant Magellan Telescope Project

The GMT project had its origin in a series of discussions between 2000 and 2003 among the original Magellan partners and interested parties outside the Magellan community. A number of groups were considering the scientific capabilities of a next generation large telescope and a variety of technical approaches to apertures beyond ten meters were being considered. In Astronomy and Astrophysics in the New Millennium, the NRC report from the 2000 Decadal Survey, the scientific case for a 30m-class telescope, as it was then understood, was clearly articulated. The notional “Giant Segmented Mirror Telescope” (GSMT) described in that report was its highest priority for ground-based astronomy.

The approach to creating the large collecting area and aperture employed in the GMT draws on the outstanding image quality derived from the structured honeycomb mirrors in use on the Multiple Mirror Telescope (MMT) and Magellan telescopes. These mirrors,

GMT Science Case | Introduction 1 with their high stiffness and short thermal timescales deliver images that match the natural seeing and can be made with very fast focal ratios, enabling large fields of view while keeping the telescope and enclosure relatively small. These projects and The Large Binocular Telescope (LBT) project had demonstrated by that time that 6.5m and 8.4m aspheres could be cast and polished to high precision at the University of Arizona’s Steward Observatory Mirror Laboratory.

The GMT uses seven 8.4m diameter primary mirror segments to produce an f/0.7 primary mirror with a collecting area of 368 square meters and a resolving power, defined by the Rayleigh criterion, of 10mas at a wavelength of 1 micron. The aperture is not completely filled and thus the peak intensity in the PSF is lowered somewhat compared to a filled aperture of the same total area, but the effect is quite modest (Johns, M., 2006) and is the attenuation is only slightly larger than that resulting from the central obscuration in a typical Cassegrain telescope.

Figure 1.1 Rendering of the GMT structure showing the segmentation of the primary mirror, the telescope mount and the pier.

In 2004 the Carnegie Institution, the University of Arizona, Harvard, and Smithsonian started the GMT project. The University of Texas at Austin and Texas A&M University joined the project in 2006, followed in 2008 by the Korea Astronomy and Space Science Institute (KASI), the Australian National University, and Astronomy Australia Limited.

2 Introduction | GMT Science Case

That same year, the GMT founding institutions signed a partnership agreement and formed an independent not-for-profit corporation to manage the development, construction, and operation of the GMT. The University of Chicago joined the project in 2010.

Table 1.1 Basic properties of the GMT

Property Value Notes Collecting Area 368 square meters 7 x 8.4m diameter segments Angular Resolution 10 mas at 1m Rayleigh criterion – 1.2/D Optical Prescription Aplanatic Gregorian -- Final Focal Ratio f/8 -- Focal Plane Scale 0.997 arcseconds per mm -- Wavelength Range 0.32 – 25m Al coatings baselined Field of View 20 arcminute diameter With wide-field corrector

The GMT will be located at Las Campanas Observatory (LCO) in Chile, at the edge of the great Atacama Desert. The Project surveyed three candidate sites at LCO, while monitoring the Magellan site as a reference point. After three years of testing and comparison with historical data from the Magellan site survey, operator logs at Magellan, and the 1m and 2.5m telescopes, the GMT Board selected Las Campanas Peak as the GMT site. At an altitude of 2550 meters, Las Campanas Peak is the highest point on the LCO property. Its native seeing is indistinguishable from that at the Magellan site, the median FWHM at 500nm at the zenith being 0.62 arcseconds, as measured by a differential image motion monitor. LCO has outstanding weather statistics and is free from light pollution and any significant threat of future light pollution. The two 6.5m Magellan telescopes produce outstanding images and have demonstrated that Las Campanas is a world-class site.

1.3 Structure of the GMT Science Case

The science case presented in this document is composed of three parts. First, later in this introductory chapter we consider the discovery space opened by the GMT, focusing on gains in sensitivity and angular resolution. The principal discussion then proceeds in the following chapters, detailing how the GMT can be used to address contemporary problems in astronomy and astrophysics. This is naturally where the bulk of the effort in developing the scientific rational for the facility is focused. We recognize that many of the important contributions from scientific facilities, and astronomical observatories in particular, come from unanticipated discoveries of new phenomena and processes. Examples in modern astronomy abound. The discoveries of quasars, pulsars, dark matter, Gamma Ray Bursts (GRBs), and other phenomena arose when new windows on the Universe were opened by technological advances. Many of these discoveries (e.g. quasars, GRBs) were made possible by multi-wavelength observations using a range of facilities. Finally, near the end of this book we consider the potential for scientific

GMT Science Case | Introduction 3

synergies with extant and planned facilities, including those working in other parts of the electromagnetic spectrum (e.g. radio, Far-IR/sub-mm, gamma rays) as well as those working in large-area and temporal survey domains. Naturally there is some uncertainty in making these connections as the precise mix of facilities available during the first decade of GMT operations is unknown, and yet-to-be-discovered phenomena may well drive new and unanticipated synergies.

The science case for the GMT and any other multi-use facility with either a long development or lifetime is naturally an evolving document. This is the second iteration of the GMT science case; the first being completed at the time of the GMT conceptual design review in 2006. Many areas of that discussion are now out of date. Doubtless, some aspects of the present document will be out of date by the time GMT sees first light. This document will evolve as developments dictate; we hope and expect that the basic structure will remain intact and relevant as we proceed through the construction and commissioning phases of the project, but we will revisit the science case on a regular basis.

1.4 Discovery Space Opened by the GMT

The gain from an increased aperture in seeing-limited applications is easily characterized. The number of photons collected per unit time increases as the collecting area, or D2. Thus the signal to noise per unit time increases as the first power of the diameter in background- and source-limited applications, and as the square for detector-limited environments. The time needed to reach a given signal-to-noise ratio for a fixed flux, often called the “sensitivity”, increases as the first or second power of the diameter in the background or detector-noise limited regimes, respectively. Many science applications with large telescopes are sky or background-limited. High-dispersion spectroscopy is occasionally detector-noise limited, while high signal-to-noise high-resolution spectroscopy of bright targets is often source-noise limited.

Adaptive optics (AO) allows one to concentrate the light from point or compact sources against the foreground sky. This increased image concentration provides additional gains in sensitivity as a function of aperture size. In the sky- or background-limited regime, the signal to noise ratio for a point source per unit time increases as D2, while the time needed to reach a given signal to noise ratio decreases as D4. The slightly dilute pupil provided by GMT makes this scaling somewhat more complex. Two of the powers of D 4 in the D argument arise from increased collecting area; the other two arise from improved image concentration due to diffraction. The effective diameters for collecting area and diffraction differ for GMT. Thus, when comparing to an 8m aperture, one 2 2 should consider the GMT AO sensitivity as scaling like (24.5/8) x (21.9/8) = 70.

4 This D factor is often cited as one of the primary drivers for extremely large telescopes (ELTs). Some of the most interesting applications of adaptive optics today, however, will likely not scale as D4, as they deal with objects that are partially or fully resolved at ~100mas resolution. Integral field spectroscopy of distant galaxies, for example, is one of

4 Introduction | GMT Science Case

the key science drivers for ELTs. While distant galaxies have a significant flux in compact structures, they also have significant flux in resolved structures at HST resolution and so one will not achieve the same gains in sensitivity realized for point 3 sources. We expect gains in speed that will scale as roughly D in this case.

Figure 1.2 Discovery space opened by gains in sensitivity. We compare the 5 depths in an hour of integration for current 8m apertures (tops of bars) with the depths that will be achieved by the GMT (bottoms of bars) across its entire operating wavelength range. Seeing-limited applications are shown in blue, while AO applications are shown in red. Spectroscopic limits are indicated by the curves in the visible and near-IR. Sensitivities are shown as micro-Janskys (left) and AB magnitudes (right).

There are regimes where ELTs will deliver gains in sensitivity that scale as a higher power of the aperture diameter. These include crowded fields and contrast-limited applications. Imaging of partially resolved stellar populations in crowded regions will benefit greatly from the increased image concentration for point sources and the reduced crowding and confusion noise associated with the unresolved component. Imaging of exoplanets near bright stars will also benefit from increased aperture both through concentration of the faint exoplanet signal and the reduced PSF wings from the parent star. Gains as steep D6 or steeper have been posited for this type of application. Over the next few years we will learn how this applies in practice as 8 - 10m high-contrast AO systems are deployed.

In Figure 1.2 we illustrate the discovery space opened by the GMT in terms of gains in sensitivity. In this context we consider sensitivity to mean the depth achieved at a fixed signal-to-noise ratio in a fixed time as a function of wavelength. In Figure 1.3 we illustrate the discovery space opened by the increased angular resolution offered by the GMT compared to current generations of telescopes.

GMT Science Case | Introduction 5

Figure 1.3 Angular resolution discovery space opened by the GMT using adaptive optics. The red bar shows the difference between the angular resolution of 8m diffraction-limited AO systems and the GMT AO system, as defined by the Rayleigh criterion. We also show lines for 3 and 5/D for contrast limited AO applications. The linear scale corresponding to the angular scale on the left is shown on the right for distances appropriate to exoplanets (100pc) and distant galaxies (z = 1) in AU and kiloparsecs, respectively. Seeing-limited and ground-layer adaptive optics resolution does not scale with aperture and are shown as blue lines for reference.

Adaptive optics also opens up a discovery space in spatial resolution (Figure 1.3). The gains in angular resolution simply scale as the diameter of the aperture. The factor of three increase in resolution compared to an 8m aperture opens up a number of interesting areas of parameter space. The GMT will dramatically increase the volume of space over which one can image the sphere of influence for massive black holes. Similarly, the mass range that one can probe for central black holes in nearby galaxies is improved. In the case of exoplanet imaging, a factor of three reduction in the inner working angle opens up a large number of radial velocity detected planetary systems for imaging in reflected light, where only a handful are within reach today. Similarly, in the case of thermal emission the improved angular resolution will allow GMT to reach to stellar nurseries in the southern Milky Way and to probe terrestrial zones in nearby stars and protoplanetary and debris disks.

1.5 Contemporary Science Goals

The chapters that follow deal with contemporary science topics that are both of interest and relevant to the ELT user community. We have arbitrarily ordered the topics from local phenomena (e.g. star formation and exoplanets) to distant objects and the early Universe; we could have just as well put them in the opposite order. The topics covered are:

6 Introduction | GMT Science Case

 Star, planet, and disk formation  Extrasolar planetary systems  Stellar populations and chemical evolution  Galaxy assembly and evolution  Fundamental physics  First light and reioinzation

In each of these chapters we focus on a small number of topics of interest to illustrate where the GMT can have a high impact. Examples include using its great collecting area to probe abundance patterns in the most metal poor stars, using its increased angular resolution and sensitivity to measure dynamical masses of young galaxies and using its sensitive near-IR spectroscopy to detect Lyα and HeII emission from galaxies in the first few hundred million years after the Big Bang.

By focusing on only a few questions in each area, we necessarily leave unaddressed a number of interesting questions. This is unavoidable if we are to keep the document to a reasonable length. We believe that this does not weaken the justification for, or definition of, an ELT.

1.6 Scientific Synergies

In addition to these purely science driven discussions we have added two chapters that deal more with the techniques by which astronomical investigations are carried out today. These include addressing the time domain and transient phenomena and a discussion of scientific synergy with other facilities. These two areas represent an evolution in the way we do astronomy. Young scientists in our field no longer characterize themselves as “radio”, “optical” or “x-ray” astronomers to the degree that the previous generation did. Scientists now focus on particular areas of scientific interest and avail themselves of the full range of techniques and facilities needed to address problems of interest. Scientific synergies among facilities operating in different wavelength domains or survey parameter space allow us to bring many more tools to bear on problems that naturally span a range of energies and spatial scales. These are explored in some detail in Chapter 9.

One of the last uncharted frontiers in astronomy is the time domain. A number of small- scale experiments are exploring time-domain astronomy to a far greater degree than possible in the past. The Large Synoptic Survey Telescope (LSST) will transform this field with its unprecedented combination of areal coverage, depth and cadence. In Chapter 9 we consider some of the synergies between LSST and the GMT.

1.7 GMT Instrument Candidates

In much of the science discussion in the chapters that follow, the authors make specific references to GMT instruments. These references allow the authors to illustrate specific examples of where the GMT can address contemporary science questions.

GMT Science Case | Introduction 7

Table 1.2 Candidate first generation instruments

Name Capability Wavelength Range AO Mode G-CLEF Visible Echelle 0.35 – 1.0 Seeing-limited GMTNIRS Near-IR Echelle 1.2 – 5.0 Diffraction-limited GMACS Visible MOS 0.35 – 1.0 Seeing-limited NIRMOS Near-IR MOS 0.85 – 2.5 Ground-layer AO GMTIFS Integral Field Spec 0.9 – 2.5 Diffraction-limited TIGER mid-IR planet imager 1.5 – 10.0 NGS diffraction-limited MANIFEST Fiber-Feed system -- Instrument dependant

In Table 1.2 we tabulate the six instruments and one facility system that are currently under development by the GMT partners. We do not expect that all six of these instruments will be ready by first-light. A process is in motion that will sequence the development of instruments and identify which of these six will constitute the first generation, and which may be included in the second generation. The latter will be developed during the last few years of construction and first few years of GMT operations. Below we briefly summarize the salient properties of each instrument.

G-CLEF: The GMT CfA-Carnegie-Chicago Large Earth Finder is a precision radial velocity spectrograph that also provides a general high-resolution visible spectroscopic capability. G-CLEF will reside in a gravity invariant and temperature controlled environment on the azimuth disk of the telescope. An optical and fiber relay system will transfer light from the telescope to G-CLEF and will scramble the light within the seven sub-apertures in the process. The instrument will support several observing modes with a range of spectral resolutions and input aperture sizes.

GMTNIRS: The GMT Near-IR spectrometer is a 1.2 - 5 micron echelle optimized for studies of young stellar objects, debris disks, and protoplanetary systems. It will use Silicon immersion gratings to achieve high spectral resolution in a compact format. Using slits well matched to the diffraction-limited image size from a single segment, GMTNIRS will deliver R = 100 – 120k spectra over all of the atmospheric windows in a single observation. This represents an enormous gain in observing efficiency compared to current spectrographs that sample only a fraction of a band in a single setting.

GMACS: The GMT visible multi-object spectrograph uses field and wavelength multiplexing to deliver an enormous A- combination. A tent mirror directs an 8 x 18 arcminute field of view to four collimators that are followed by dichroics that feed red- and blue-optimized cameras. The full instrument will deliver moderate resolution (R = 1500 - 5000) spectroscopy over a very large field of view over the full visible (0.35 - 1.0 micron) region of the spectrum. The A product for GMACS is ten times that of DEIMOS on Keck and 1.5 - 2 times that of any multi-object spectrograph under discussion for other ELT projects.

8 Introduction | GMT Science Case

NIRMOS: The GMT Near-IR Multi-Object Spectrograph uses cold slit masks and volume phased holographic gratings to provide a highly efficient moderate resolution (R = 3000) spectrograph with a field of view of 6.5 x 6.5. Cold slit plates can be swapped out during the day using a gate value to isolate the slit mask chamber. NIRMOS can be used in conjunction with the facility ground-layer AO (GLAO) system to increase its sensitivity and image quality. The primary science application for NIRMOS is spectroscopy of galaxies at intermediate and high redshifts.

GMTIFS: The GMT integral field spectrograph builds on the legacy of the NIFS instrument on Gemini. It uses an image slicing micro-mirror system to reformat the focal plane onto the detector. Multiple spaxel scales are possible; all feed an R = 5000 spectrograph that can cover any one of the J, H, or K-bands in a single setting. The instrument also has an imaging channel that will cover the laser tomography AO (LTAO) field with critical sampling in the J-band.

TIGER: The GMT thermal IR imager targets exoplanets, debris disks and low mass stars, but will also provide a powerful capability for studying AGN and starburst nuclei. The camera operates from the H-band through the 10 micron band and will employ a variety of techniques to provide high contrast for exoplanet imaging.

MANIFEST: The GMT has an unusually large field of view for an ELT. While some of the proposed instruments use a significant portion of the field, none make full use of the 20 diameter. The GMT facility fiber system provides a means to observe multiple targets over the entire field of view with one or more of the spectrographs. MANIFEST can, in principle, feed GMACS, NIRMOS, and G-CLEF. The precise multiplexing gains vary for each spectrograph depending on the available slit length. The MANIFEST concept uses “Starbugs” – self-motile fiber heads deployed on a glass plate. MANIFEST offers a means by which GMT can be optimized for high A survey science without reimaging the full focal plane.

1.8 Adaptive Optics System

The GMT design incorporates adaptive optics into the telescope through the use of adaptive secondary mirrors. Adaptive secondary mirrors have been developed for the MMT and LBT AO systems and produce outstanding diffraction-limited images with high throughput and low thermal background. The European Southern Observatory (ESO) is developing an adaptive secondary for the VLT and employs an adaptive flat mirror (M3) in the E-ELT design. The GMT secondary will be segmented one-to-one with the primary mirror. Each 1.09m diameter secondary mirror will have a similar actuator count and pattern as the LBT secondary mirrors.

The GMT project expects to support multiple AO modes in the early years of operations (Table 1.3). The simplest mode is on-axis natural guide star AO. In this mode the target, or a nearby bright star, provides the wavefront reference. High Strehl ratios can be achieved in this mode over small fields of view. The principal AO mode is laser

GMT Science Case | Introduction 9

tomography AO. This mode relies on laser guide beacons tuned to the atmospheric sodium layer. Moderate Strehl ratios can be achieved with high sky coverage in this mode. The LTAO mode will be the primary means for observing faint targets (e.g. galaxies, Active Galactic Nuclei (AGN), Kuiper Belt Objects (KBOs)) and extragalactic sources.

Table 1.3 First generation adaptive optics modes

Mode Field of View† Angular Resolution Natural Guide Star 20 arcseconds Diffraction-limited 24.5m Laser Tomography   Ground-Layer Correction Up to 8 diameter Factor of 2 – 4 improved seeing

† The useful field will vary with atmospheric condition

The GMT adaptive secondary is particularly well suited to ground-layer correction. The Gregorian prescription of the AO secondary mirror provides a conjugate near the height of the turbulent boundary layer, approximately 150 meters above ground level. In the GLAO mode the image size is reduced but a seeing-like profile is preserved. Reductions in FWHM of a factor of 2 - 4 have been achieved over modest fields (1 arcminute) and gains of 50% - 200% over larger fields in median conditions are expected for the GMT GLAO system. Several groups (e.g. SOAR, Gemini) are developing ground-layer systems at present so we will know more about the performance of this AO mode in the near future.

1.9 Summary

In the chapters that follow we will demonstrate how the large gains in sensitivity, resolution and performance offered by the GMT can be used to advance astrophysics. The authors of the various chapters draw upon the basic capabilities and, often, specific instruments and subsystems in laying out their science case. Many of the science applications rely on the large gain in collecting area offered by the GMT primary; others primarily draw on the improved angular resolution and AO system. The most powerful applications rely on both the giant collecting area and high angular resolution to probe areas of parameter space that are out of reach with present facilities.

References

Johns, M. 2006, Ground-based and Airborne Telescopes. Edited by Stepp, Larry M. Proceedings of the SPIE, Volume 6267, pp. 626729.

10 Introduction | GMT Science Case

1.10 List of Contributors

Many individuals have contributed to this document and the background work that supported it. Below we list the names and institutions of the primary authors, those that have supplied key figures or sensitivity estimates or have otherwise made direct contributions to this document. Undoubtedly we have missed a few who have made indirect or direct contributions, for which we apologize.

Name Institution Name Institution

Sean Andrews Smithsonian Joe Hora Smithsonian

Jacob Bean Chicago Dan Jaffe UT Austin

Jamie Bolton U. Melbourne Sang Chul Kim KASI

Warren Brown Smithsonian Rich Kron Chicago

Hsiao-Wen Chen Chicago Patrick McCarthy GMTO

Gary Da Costa ANU Peter McGregor ANU

Darren DePoy Texas A&M Andrew McWilliam Carnegie

Megan Donahue Michigan State Casey Papovich TAMU

Daniel Fabricant Smithsonian Dimitar Sasselov Harvard

Xiaohui Fan Arizona Rob Sharp ANU

Steve Finkelstein TAMU/UT Austin Scott Sheppard Carnegie

Duncan Forbes Swinburne Josh Simon Carnegie

Anna Frebel Smithsonian/MIT Andrew Szentgyorgi Smithsonian

Karl Gebhardt UT Austin Chris Tinney U. NSW

Mike Gladders Chicago Alycia Weinberger Carnegie

Phil Hinz Arizona Craig Wheeler UT Austin

GMT Science Case | Introduction 11

2 Formation of Stars and Planetary Systems Alycia Weinberger, Phil Hinz, Dan Jaffe, Sean Andrews and Scott Sheppard

2.1 Introduction

The theory of stellar evolution is one of the great successes of 20th century science. From the faintest brown dwarfs to the most luminous blue supergiants, the theory explains how galaxies transform their gas into luminous matter. Star formation begins with molecular clouds, where discrete clumps of gas and dust gravitationally collapse to form protostars. Conservation of angular momentum during the collapse results in gas and dust settling into circumstellar and protoplanetary disks, in which planets may coalesce and grow. As the forming star becomes more luminous, develops its magnetic field, emits energetic photons, and launches a jet, it changes the chemistry and dynamics of its circumstellar disk and the conditions for planet formation within. Thus, star and planet formation occur in concert within the confines of the protostellar disks engulfing these systems (Williams & Cieza 2011).

A wide diversity of extrasolar planets and planetary architectures has been revealed by planet detection experiments such as the California-Carnegie Planet Search, Kepler, HAT, and MicroFun among others. Known planets today have masses that range from a few to 10,000 Earth masses, and locations that range from a few hundredths of an AU to tens of AU from their parent stars. Understanding how this remarkable panoply of systems formed is one of the great challenges facing astrophysics. It is believed, for example, that the physical properties of, and processes occurring within circumstellar disks must determine system architectures. But constraining theoretical models for the distribution of planetary orbits and masses requires better statistics on the locations and timescales of planet formation, planetary migration rates (planets presumably form at a range of initial locations but migrate through gaseous disks), and disk dissipation times.

The fundamental mechanism by which gas giant planets in particular form is still disputed. The “core-accretion” model, wherein the cores of planets build up slowly by collisions and then rapidly accrete gas, takes millions of years—comparable to the lifetime of protoplanetary disks (Pollack 1984). Moreover, in this model, planets must form in a relatively narrow range of distances from their stars—far enough out to take advantage of higher surface densities afforded by condensed ices, but close enough for short orbital times to allow accretion of substantial mass. While it has its merits, this “traditional” mechanism has trouble producing massive planets around low-mass stars, though several are observed. In a competing model, the “gravitational instability” mechanism, disks fragment into self-gravitating cores that collapse to become giant planets, thus forming planets quickly (Boss 1997). Disks must be sufficiently massive and cold enough to become unstable and may form planets at large distances from their central stars. Measurements of disk density are needed to determine if such conditions are met and to ascertain the relative importance of the two processes.

12 Formation of Stars and Planetary Systems | GMT Science Case

Solids and gases in regions of planet formation (i.e. the inner tens of AU of circumstellar disks) are heated by the central star and reach temperatures of ~100 - 1000K. They therefore emit strongly at near- and mid-infrared wavelengths. Using near-infrared light to probe disks on scales of a few AU for stars in the solar neighborhood and beyond requires imaging with angular resolution of ~10 – 50mas, and an aperture of ~25m operating at the diffraction-limit. GMT’s natural guide star AO system will be well suited to these studies and its southern hemisphere location is an ideal location for observing the closest massive star-forming regions.

2.1.1 Synergy with other instruments

When studying star and planet formation, GMT will provide synergy with the Atacama Large Millimeter Array (ALMA) in Chile. While ALMA will excel at measuring dense disk midplanes, cold dust and outer disks as well as molecular lines, GMT will be capable of studying warm dust and inner disks as well as molecular lines that trace warm gas. GMT will also complement the James Webb Space Telescope’s (JWST) tremendous sensitivity for low-resolution spectroscopy of young stellar environments with four times the spatial resolution and its high contrast and high spectral resolution capabilities.

The GMT will further our understanding of the connections between stars, their disks and the planets that can form within them by observing the key components directly. Planets down to the super-Earth scale will be detectable through disk perturbations. With its unprecedented sensitivity, GMT will also be able to detect new molecules and ices (including those involving water and organic materials) in protoplanetary disks. These data will enable a census of the complex chemistry in clouds and disks that may play a role in the development of life.

The southern sky that passes over Las Campanas is particularly rich in young star formation regions. A few close examples of protoplanetary disks exist at 50 - 100pc from the Sun, but the nearest regions of active star formation and high concentrations of young stars with high space densities lie at distances beyond 130pc. GMT will be able to take advantage of this wealth of targets and image many more disks thus providing a deeper understanding of how stars of different masses form, how they influence their environments, and how they enable the formation of planets. The specific scientific contributions related to star and subsequent planet formation are outlined in detail below.

2.2 Astrophysics of Young Stars

Stars form and evolve hand-in-hand with planets, and stellar properties including mass, rotation, and magnetic field strength control their co-evolution. Therefore, we cannot expect to understand the evolution of the disk and its consequences for planet formation unless we understand the stars that host these processes. GMT will deliver the capacity for dramatic advances in stellar astrophysics and circumstellar disk science by addressing questions such as: What changes in the birth characteristics of stars (such as angular

GMT Science Case | Formation of Stars and Planetary Systems 13

momentum and magnetic fields) affect their later evolution? What correlations exist between stars and the masses and compositions of disks? And how does ongoing accretion affect stellar structure?

 Disks

Disks and envelopes around protostars obscure our view of central stars through a combination of extinction and veiling, i.e. obscuration due to the disk itself and to accretion from the disk onto the star. For example, there are approximately 300 young stars in the rho Ophiuchus cloud core at ~130pc, region L1688 (Wilking, Gagne & Allen 2008). Stars brighter than K = 11 can be observed with today’s large ground-based telescopes, but only about 60% of the total sample and 5% of the youngest sources (i.e. those with optically thick disks and/or envelopes) are currently accessible. Therefore, the most interesting samples of young stars in this cluster—those whose early disk evolution we would most like to characterize—are inaccessible today. GMT will penetrate this obscuration and make spectroscopic measurements of the protostar that reveal stellar properties. Indeed, GMTNIRS could observe each and every star currently known in the region L1688 in under an hour of integration time.

 Ages

It is widely thought that embedded stars are younger on average than their unobscured counterparts in the same regions, but this paradigm has yet to be tested. GMT’s high spectral resolution capabilities will enable the collection of multiple, simultaneous astrophysical measurements (Figure 2.1), including accretion diagnostics, jet/wind emission lines, and detection of photospheric lines against the continuum generated by the disk so that the spectral types and gravities of the stars can be determined and thereby their ages, as compared to stellar evolution models.

 Magnetic Fields

It is also widely believed that stellar magnetic fields thread the inner disk and are essential for driving accretion onto the protostar and removing angular momentum from the disk by driving jets. However, to date, a stellar magnetic field has only been measured in one embedded protostar (Johns-Krull et al. 2009). Again, with its exceptional spectral resolution, GMT will be able to measure stellar magnetic fields using Zeeman broadening of lines (such as TiI) at spectral resolutions > 30,000.

 Winds

X-ray and UV radiation from central stars drive photoevaporative winds and ultimately limit the time available to form planets around young stars. This is a key form of star-disk interaction that leads to the removal of gas (the primary raw material of the disk) over timescales of a few Myr. Adaptive optics coupled with high spectral resolution enables “spectroastrometry”, the spatially resolved study of line kinematics (Pontoppidan et al.

14 Formation of Stars and Planetary Systems | GMT Science Case

2011). GMTNIRS can apply this technique to molecular lines such as CO and water to trace the interactions between a young star and its disk, and in particular, distinguish between the Keplerian rotation of the disk and photoevaporative winds thereby revealing mass-loss processes in young stellar/protoplanetary systems.

H2 O OH

CO Fundamental

HeI

Flux Pa 

H Lines

Br  Ti Zeeman splitting

1 2 3 4 5

Wavelength ( m)

Figure 2.1 Spectrum of a classical T Tauri star (TW Hydrae; Vacca & Sandell 2011) showing some of the variety of photospheric and disk diagnostics across the IR spectrum. Hydrogen and helium emission lines probe disk accretion physics; Zeeman splitting measures stellar magnetic fields; and molecular emission lines reveal disk kinematics and temperature structure. Regions of strong telluric absorption are shaded.

 Jets and Knots

Interactions between stars and disks drive high-speed jets and a bipolar molecular outflow. Although we have a fairly clear picture of large-scale processes in the outflow, we know little about the processes that turn accretion energy into a collimated jet (Ray et al. 2007). In the nearest star forming regions, diffraction-limited imaging in the infrared with GMTIFS will enable the first spatially resolved observations of jets close to the central star. Integral field spectroscopy in Brα, Brγ, HeI 1.08μm and 2.06μm, and numerous metallic lines will probe the geometry, density, and temperature of gas in the region of jet formation at a few to a few tens of AU from the central star. Time-resolved high resolution profiles of these near-IR and mid-IR lines will shed light on the origins of jets by probing in-falling and ejected material closer to the star, including material near the inner disk and at the magnetosphere where current theory suggests jets are launched. Velocity resolutions of 20km/sec could spectrally resolve rotation within 10AU of the jet launch-point for solar-mass stars. The combination of spectral diagnostics of accretion with the imaging of inner disks will yield tests of magnetospheric accretion and jet formation.

GMT Science Case | Formation of Stars and Planetary Systems 15

Structure in the out-flowing material (and knots in particular) reveals irregularities in accretion, and turbulent flow within the jets themselves. Knots typically move out at ~100km/s, thus for young stars at typical distances of ~150pc, they will move by ~20mas, or one GMT resolution element at K (2.2μm), in under 2 months. If disk accretion events, as monitored by changes in unresolved emission lines, lead to changes in the jet launch rate or velocity, GMT will be able to observe knots as they appear after major disk accretion events. This timing will elucidate the mechanisms of the jet’s collimation and launch. Current observations cover only a handful of the brightest sources; GMT’s great sensitivity will provide the first large samples of high spatial and spectral resolution observations of jets from protostars.

2.3 The Initial Mass Function and the Potential for Planets

Understanding the origin of the initial mass function and the prevalence of binary and multiple systems of stars and sub-stellar objects remains one of the most important unsolved problems in galactic astronomy. In addition to providing fundamental constraints on theories of star formation, determining the shape of the initial mass function as a function of chemical composition, star formation rate, and galactic environment is essential for understanding the formation and evolution of galaxies. Similarly, assessing the capability of stars of varying masses and in a variety of environments to form planets is important to our understanding of planetary systems and will require observing stars and brown dwarfs in regions of low- and high-mass star formation and in single and multiple star systems. Studying the evolution of disks around high- and low-mass stars in clusters will reveal whether planet-forming disks can survive around the full range of host-star types, and for how long.

The GMT will enable the first systematic and unbiased surveys of multiplicity (the frequency of bound systems of two or more stars) at ages of ~1 - 10Myr for star clusters within 500pc. These comprehensive measurements of the multiplicity fraction as a function of mass ratio and separation can then be used to discriminate between fragmentation modes of star formation. They will also provide a wealth of information on how multiplicity is affected by local cluster environments (e.g., stellar density) that can then be compared to the population of multiples in the solar neighborhood (e.g. Patience et al. 2002). With its high contrast, GMT’s TIGER coronagraph will be able to identify all companion objects well below the brown dwarf mass limit with projected separations as small as ~7AU (50mas at d = 150pc) in the L'- or M-band. Finally, GMT will be able to compile complete multiplicity censuses in nearby young clusters of pre-main-sequence stars.

A more detailed line of work for GMT will be to glean dynamical masses for newly detected companions from their orbital motions. With its small inner working angle, the GMT+TIGER combination will enable entirely new generations of astrometric studies for young, close-separation binary systems in nearby clusters. The orbital periods of stellar (or star-brown dwarf) binaries with ~10AU separations are on the order of 30 years. With the high-contrast capabilities of the GMT, high S/N astrometric monitoring would show substantial orbital motion on year-long timescales for multiples with favorable viewing

16 Formation of Stars and Planetary Systems | GMT Science Case

inclinations. For the closest binaries, near-infrared high-resolution spectroscopy can reveal the radial velocities of each component. Together, astrometry and velocity data will permit measures of stellar mass, independent of the distance to the sources. These dynamical measurements of pre-main-sequence masses will be of extraordinary value— providing a fundamental calibration of young stellar evolution models, which are currently the only means for inferring masses, ages and disk lifetimes. In addition, if individual stars display gas emission lines (e.g. of CO) in their disks, the mass of the star can be measured directly from the disk rotation curve.

 Brown Dwarf Stars

Brown dwarfs, sub-stellar mass objects that cannot sustain nuclear fusion, inhabit the mass range between stars and giant planets. Characterization of their space density and properties will have important implications for our understanding of stellar convection, the formation of massive planets and mass-to-light ratios in stellar populations.

At the distance to the nearest regions of ongoing star formation, such as Ophiuchus, -4 models suggest that a single object of mass 2 - 3MJup, and a luminosity of 10 LSun would have an I-magnitude of 26 and H-band magnitude of 19 (Chabrier et al. 2000). However, few tests of these theoretical models exist for young brown dwarfs as poorly known initial conditions lead to models fraught with uncertainties. While young brown dwarfs may be first identified in wide-field imaging surveys on smaller telescopes, they must be confirmed and studied spectroscopically. GMT will be able to study the physical conditions of candidate brown dwarfs with high-resolution infrared spectroscopy and obtain astrometric imaging and spectroscopic orbits of close binaries to measure masses. Impressively, in only 1 hour, GMTNIRS could obtain R = 50,000 spectra of these free- floating planetary mass objects.

In spite of over 40 years of study, it is still not known if the initial mass function is universal. Clouds with dense star clusters may form many more brown dwarfs and more massive stars per unit mass than low-density clouds with loose stellar associations. At the low-mass end, searches for brown dwarfs have been limited to a few nearby clusters and are therefore plagued by poor statistics. To understand the final Galactic population of low mass objects, we must study multiple star forming environments to determine where brown dwarfs form, and in what numbers.

2.4 How Circumstellar Disks Form Planets

Disks with masses sufficient to form a planetary system like the Solar System have been detected around more than 80% of Sun-like stars at young ages (~1Myr) but are essentially non-existent for stars older than 10Myr (Strom et al. 1989, Williams & Cieza 2011). Unfortunately, despite 20 years of work and the thousands of stars known to harbor disks, only a few tens of disks have been spatially resolved. This is largely due to the combined observational challenges posed by the high contrast between protostars and disks and the large distances to the stars.

GMT Science Case | Formation of Stars and Planetary Systems 17

Planets can induce local temperature and density structure in disks such as gaps, warps, and resonant rings, making disk-science ripe for progress from the higher angular resolution imaging that GMT will deliver. Kinematically resolved emission lines can be used to probe the composition and structure of spatially unresolved disks. GMT will study the physics of disks and interactions with forming planets with a combination of spatially resolved imaging and spectroscopy.

If planets form early by gravitational instability (rather than by core accretion) the GMT will be able to image them around very young stars and subsequently study them spectroscopically. For example, the recently detected 8MJup planet around the 12Myr old star β Pic has an L-band apparent magnitude of 11 (Lagrange et al. 2010). Figure 2.2 shows that GMT would easily obtain spectra of this planet in under one hour. GMTNIRS’ high-spectral resolution will enable the determination of the molecular composition of the planet, including the abundances of important molecules such as water, methane and carbon monoxide, as well as evidence for auroral emission due to a planetary magnetic field producing H3+. When followed over multiple days or years, detection of variability can be used to infer the properties of winds and clouds in planetary atmospheres. As more planets of lower masses and fainter magnitudes are discovered closer to their stars by direct imaging at smaller separations (see Chapter 3), GMT spectra will facilitate further studies of their physical conditions. Direct measurements of the birth compositions and structures of giant planets will provide the foundation for evolution models that describe mature planets observed around nearby stars.

Figure 2.2 The spectrum of a young planet such as β Pic b can reveal the composition of the atmosphere and the presence of dusty clouds. The black bars show a simulated spectrum for a 1400K planet observed around β Pic in 1 hour at a final spectral resolution of 30,000, as it would look if the atmosphere were cloudless (dark blue model). Dusty clouds prevent seeing deep into the atmosphere and produce a spectrum easily distinguished with GMTNIRS (light blue model). Models courtesy of Travis Barman.

18 Formation of Stars and Planetary Systems | GMT Science Case

To constrain planet formation and migration via disk physics, we must use spectroscopy in conjunction with adaptive optics. The sensitivity of current 8 - 10m telescopes, however, has only been sufficient to probe disks around bright, high-mass stars and those least embedded in their parent molecular clouds. With the smaller inner working angle in the diffraction-limited AO modes and spectroscopic capabilities, the GMT will be able to study a much wider range of stars with varying properties and in a range of evolutionary states. At early times, spectra of H2, CO, water, methane, acetylene, and other transitions at 1 - 5mm, and silicate grains at 10 - 20mm probe the structure of the disk at 0.1 - 10AU enabling studies of mass-flow through the disk, the growth of grains, and the formation of organic molecules within the disk. At later times, these transitions will allow the first measures of the relative abundances of gas and dust in terrestrial and gas giant zones.

Studying young disks requires high spatial and spectral resolution. The GMT’s smaller inner working angle compared to 8 - 10m apertures will allow it to carry out observations that probe to 2AU at 1.6μm or 4AU at 3.8μm (for the typical 140pc distance of nearby star forming regions) thereby discovering large samples of young disks. High contrast (10-6) AO images using TIGER with 4AU resolution will enable the first direct views of the terrestrial and gas giant formation zones in disks around the youngest nearby stars. Thermal infrared imaging at ~10μm may further reveal temperature and structure perturbations in disks, as in Figure 2.3. Gravitational instability leads to the formation of spiral arms that may be observable in scattered light. These features can change on short timescales (< 10 years) and reveal the presence of forming planets. If planets form on 1 - 10Myr timescales, imaging observations will enable searches for the bright rings and clumps, dark bands and shadows, and warps that provide indirect evidence for planets.

GMT observations of the CO fundamental lines will probe the kinematics of warm gas and may allow separate observations of the Keplerian gas motion, and hence a direct constraint on the stellar mass and radial motions of gas such as streamlines past a giant planet in formation.

• The Snow Line

An interesting spatial scale for planet formation is the “snow line”—the demarcation separating an ice-rich outer disk, which should be capable of forming giant planets, from an ice-poor inner disk. The snow line separates the region of our Solar System containing gas giant planets from the region containing terrestrial bodies The concept of the snow line is simple in principle, but is likely more complicated in practice since the temperature structure of a disk varies not just with distance from the star but also with vertical height. One way to probe the location of the snow line is by using near-IR spectroscopy and spectro-astrometry to observe both H2O and its dissociation product OH; a distinct change in the abundances of each should appear at the snow line. A second probe is provided by observing in and out of the broad 3.3μm water ice feature. Such observations will reveal absorption and scattering of light from the central star and will be sensitive to the distance at which ice appears (Inoue et al. 2008).

GMT Science Case | Formation of Stars and Planetary Systems 19

Figure 2.3 (Top) Simulated TIGER image of protoplanetary disk inclined at 45deg to our line of sight, with a 30MEarth planet at 10AU that carves a gap in the disk (Jang-Condell 2011). The bottom left image shows this disk around a star at 100pc as imaged at 3.8μm, and the bottom right image shows the same disk around a star at 50pc as imaged at 12μm. The GMT has the power to reveal planets in formation and their interactions with the disks from which they form.

2.5 Volatile Delivery and Planetary System Architecture

After a disk has been cleared of gas through photoevaporation or planet formation, a disk of planetesimals (akin to comets or asteroids and other small Solar System bodies) must remain and interact dynamically with any planetary system. During this period—much as is expected to have been the case in the Solar System at 10 - 500Myr age—volatile-rich bodies can collide and evaporate to form a debris disk. Such dust has been found to persist to ages of Gyrs around about 10% of main-sequence stars, which suggests that planet formation progressed at least as far as the planetesimal phase in all of these systems (Trilling et al. 2008). Structure in debris disks is generated from the architecture

20 Formation of Stars and Planetary Systems | GMT Science Case

of the planetary system as the planets dynamically sculpt the reservoirs of small bodies and induce collisions in certain locations. Thus, images of dust distribution (including warps, rings, and clumps) can reveal the locations of planets too small or too cold to image directly (Stark & Kuchner 2008). With high-contrast imaging, TIGER and the GMT Adaptive Optics system can reveal these structures at the highest spatial resolution available. Figure 2.4 shows an example.

Figure 2.4 Simulated GMT/TIGER image of the disk around HR4796A at 3.8μm. Left: The Kuiper Belt-like disk at 1’’ (70AU) has been imaged at 5x lower spatial resolution with HST, but will be revealed spectacularly by GMT. Modeled here as smooth, GMT would be able to reveal any clumps or asymmetries not visible from HST. Right: A close-up of a simulated TIGER image of a purely hypothetical inner disk – a model of zodiacal-like emission 1500 times the Solar System Zodi sitting in the habitable zone of the star. GMT would not only detect such dust but also spatially resolve it. The true distribution of dust around this star depends on the architecture of its planetary system, which could be inferred from GMT images even if the planets are too small to detect directly.

Debris dust observations will allow us to sample the compositions of planetesimals that both build terrestrial planets and deliver their volatiles. Planetesimal composition plays a critical role in the process of planet formation, and also in determining a terrestrial planet’s habitability. The widely differing compositions of Solar System terrestrial planets, asteroids, gas-giant moons, and Kuiper Belt objects (KBOs) suggest that formation location and the extent of radial transport of gas and solids contribute strongly to a planet’s final characteristics. Multi-band photometry of debris disks with HST has shown that many have red colors in visible to near-infrared scattered light, potentially indicating the presence of copious organic species being delivered to any planets present. TIGER imaging and low-resolution spectroscopy in the near to mid-infrared would reveal both the signatures of organics (e.g. blue 2 - 4μm colors) and potentially reveal water or methane ice and silicate compositions. These will give the first glimpses of planetary compositions in these systems.

GMT Science Case | Formation of Stars and Planetary Systems 21

2.6 Solar System Clues to Planet Formation

While GMT will be able to observe disks around other stars, the dynamical and physical properties of small bodies in our own Solar System provide constraints on the formation, evolution and migration of our own system’s planets. The orbits of objects in the currently stable small body reservoirs have a "fossilized" dynamical imprint from the movement of the major planets. The compositions of these small bodies also serve to constrain the original composition of the solar nebula. By studying the locations and compositions of distant outer Solar System objects, GMT will reveal the early history of the Solar System and allow comparisons with exoplanetary systems.

The GMT can image to about 29th magnitude in a single 10 minute image. This translates to sizes of a few km in the Kuiper Belt. Only HST can go this deep and only in tiny (0.02 deg2) survey patches (Bernstein et al. 2004). With its wide field imaging spectrograph, GMT will detect about 50 faint outer Solar System objects per night: over ten times more per night at these small sizes than ever before discovered. GMT would then directly detect objects that are the sizes of all known short-period comets.

Understanding the small end of the Kuiper Belt size distribution will not only give insights into the formation and collisional evolution of small objects in the outer reaches of the Solar System, but will also determine if the Kuiper Belt is the only source of the Centaurs and short period comets. Only Sedna, one of the five largest objects known in the outer Solar System, has been found to have a perihelion significantly beyond the edge of the Kuiper Belt. Current surveys cannot find darker, smaller objects that may populate the region beyond 50AU. The more typical 100km or smaller objects that may lie at ~80AU are fainter than R = 26 and only discoverable by GMT in a few square degree survey to 29th mag. With this capability, GMT can contribute greatly to our understanding of the size distribution for the smallest objects in the Kuiper Belt, and whether any exist beyond the edge of the Belt at 50AU.

A direct optical imager on the GMT with a > 0.1 square degree field-of-view would have a survey power comparable to the LSST. While GMT will not cover nearly as much sky as the LSST, it will be able to probe a few magnitudes fainter to find smaller and more distant objects. The LSST survey is not designed to stack images of moving objects and thus will have difficulty imaging objects significantly fainter than about 26th magnitude. A GMT optical imager would only require a moderate pixel scale chosen so as not to saturate on the sky in a short amount of time. Only a few nights would be needed to obtain a large sample of faint objects, and GMT is the only telescope capable of determining the orbits of these objects and therefore the dynamical imprints of outer planet migration.

Of the more than one thousand known Kuiper Belt objects beyond the orbit of Neptune, only a few dozen (a sample complete to current practical spectroscopic limits of R ~ 21) are bright enough for spectral studies of their compositions (Trujillo et al. 2005). Since these larger outer Solar System objects have likely experienced significant melting and differentiation and/or atmospheric changes giving rise to highly altered surface

22 Formation of Stars and Planetary Systems | GMT Science Case

compositions, they cannot contribute significantly to our understanding of the composition of the Solar System’s primordial cloud. While future surveys such as LSST will find thousands more fainter objects, only GMT will have the power to obtain detailed surface measurements. These measurements of the smallest outer Solar System objects (likely the most primordial and least altered) will constrain the composition of the original solar nebula—its methane and water ice and organic fractions, and the elements that went into forming the planets.

Distant Sedna-type objects probably formed even further out and have been kept in cold storage for billions of years; they could be chemically very different from the known Kuiper Belt objects. Sedna, at R ~ 21, required one good night of 8-meter telescope time to obtain a spectrum with sufficient signal-to-noise to demonstrate that it was one of the few objects to have methane ice on its surface (Barucci et al. 2005). Since new Sedna- type discoveries will likely be several magnitudes fainter, only GMT and JWST will be able to determine their surface compositions.

The important volatiles (e.g. H2O, CH4, NH3, HCN, CO, CO2, and N2) to observe on outer Solar System objects are all detectable in the infrared between about 1.2μm and 2.3μm. Low resolution spectra can further determine if any water ice is crystalline, a state produced during resurfacing events, possibly from cryovolcanism from a subsurface ocean (Jewitt and Luu 2004). The high spatial resolution of the GMT will also allow for surface heterogeneity to be observed on the surfaces of the largest few outer Solar System objects.

2.7 Summary

Star and planet formation are linked through the evolution of disks. Many key question concerning the formation of planetary systems, their diversity and their evolution remain unanswered. The GMT, working in synergy with ALMA, JWST and other long- wavelength and survey facilities can provide uniquely powerful tools for exploring the evolution of protoplanetary and debris disks and their impact on the structure of planetary orbits and compositions. Adaptive optics both in the near- and mid-IR, but particularly at wavelengths where heavy extinction can be overcome, will allow the GMT to observe young planets while they are still enshrouded in their birth environs. As accretion gives way to outflow, the GMT can probe stellar jets at their origin and provide unique diagnostics of the outflow process.

A better understanding of the connection between star and planet formation can help lay a foundation for a deeper understanding of exoplanets, the subject of the next chapter.

GMT Science Case | Formation of Stars and Planetary Systems 23

References

Barucci, M. A., Cruikshank, D. P., Dotto, E., Merlin, F., Poulet, F., Dalle Ore, C., Fornasier, S. & de Bergh, C. 2005, A&A, 439, L1 Bernstein, G. M., Trilling, D. E., Allen, R. L., Brown, M. E., Holman, M. & Malhotra, R. 2004, AJ, 128, 1364 Boss, A. P. 1987, Science, 276, 1836 Chabrier, G., Baraffe, I., Allard, F. & Hauschildt, P. 2000, ApJ, 542, 464 Inoue, A. K., Honda, M., Nakamoto, T. & Oka, A. 2008, PASJ, 60, 557 Jewitt, D. & Luu, J. 2004, Nature, 432, 7018 Johns-Krull, C. M., Greene, T. P., Doppmann, G. W. & Covey, K. R. 2009, ApJ, 700, 1440 Lagrange, A.-M., Bonnefoy, M., Chavin, G., Apai, D., Ehrenreich, D., Boccaletti, A., Gratadour, D., Rouan, D., Mouillet, D., Lacour, S. & Kasper, M. 2010, Science, 329, 57 Patience, J., Ghez, A. M., Reid, I. N. & Matthews, K. 2002, AJ, 123, 1570 Pollack, J. B. 1984, Ann. Rev. Astro. Astrophys., 22, 389 Pontoppidan, K. M., Blake, G. A., & Smette, A. 2011, ApJ, 73, 84 Ray, T., Dougados, C., Bacciotti, F., Eislöffel, J. & Chrysostomou, A. 2007, in Protostars and Planets V, ed. B. Reipurth, D. Jewitt and K. Keil, pp. 231-244 Stark, C. C. & Kuchner, M. J. 2008, ApJ, 686, 637 Strom, K. M., Strom, S. E., Edwards, S., Cabrit, S. & Skrutskie, M. F. 1989, AJ, 97, 1451 Trilling, D. E., Bryden, G., Beichman, C. A., Rieke, G. H., Su, K. Y. L., Stansberry, J. A., Blaylock, M., Stapelfeldt, K. R., Beeman, J. W. & Haller, E. E. 2008, ApJ, 674, 1086 Trujillo, C., Brown, M. E., Rabinowitz, D. L. & Geballe, T. R. 2005, ApJ, 627, 1057 Vacca, W. D. & Sandell, G. 2011, ApJ, 732, 8 Wilking, B. A., Gagné, M., & Allen, L. E. 2008, in Handbook of Star Forming Regions, Volume II, ed. B. Reipurth, pp. 351-448 Williams, J. P. & Cieza, L. A. 2011, ARAA, 49, 67

24 Formation of Stars and Planetary Systems | GMT Science Case

3 The Properties of Exoplanetary Systems Chris Tinney, Phil Hinz, Dimitar Sasselov, Dan Jaffe, Jacob Bean

3.1 Introduction

Our view of the local Universe has changed dramatically over the last two decades. Our own Sun is no longer the only star known to host planets—we now know of a startling diversity of worlds orbiting many hundreds of stars. Our models for how exoplanetary systems form and evolve have been significantly revised in light of the rapid pace of these discoveries. And that pace is not likely to slow down at any point in the near future.

Over the next decade or so astronomers face the remarkable prospect of being able to detect and study the first habitable planets orbiting stars in the solar neighborhood. Right now, the Kepler mission is identifying thousands of new transiting planets (Borucki et al. 2011) and by the time its mission is complete, our understanding of the statistical properties of planetary systems will have greatly expanded. Just one year into its mission, Kepler is revealing that planets as small as a few Earth radii orbit at least 10% of stars (Howard et al. 2011). Kepler’s impact stands to be so revolutionary that we can be confident that future space-based transit searches will discover new Earth-like transiting planets before the end of the first few years of GMT’s operations1. These small planets will join a coterie of larger planets discovered from the ground.

Direct imaging techniques are also beginning to produce the first exoplanetary detections (Marois et al. 2008, 2010; Kalas et al. 2008) and over the next decade these methods promise to find more and more giant planets in the outer regions of planetary systems. We can also expect that over the next 10 years, Doppler planet searches will continue to push their precision limits to lower masses, identifying terrestrial-mass planets with orbits of under a year (e.g. Howard et al. 2010, Wittenmyer et al. 2011b), while also determining the frequency with which Jupiter-mass planets orbit Sun-like stars in orbits comparable to those of gas giants in our Solar System (e.g. Wittenmyer et al. 2011a).

These developments promise a dynamic and exciting landscape in exoplanet science when GMT starts operations. The GMT will make unique contributions through its ability to identify and characterize habitable worlds orbiting the nearest stars; to probe the atmospheres and compositions of planets both large and small; and to image the outer planets, and potentially even the rocky inner habitable planets that may orbit our Sun’s stellar neighbors.

1 For example, TESS (space.mit.edu/TESS/Mission_presentation/Welcome.html) was selected in October 2011 for Concept Study evaluation as a NASA Explorer mission.

GMT Science Case | The Properties of Exoplanetary Systems 25

3.1.1 Current models of Planet Formation

Two paradigms currently dominate planetary formation models: core accretion (see e.g. Ida & Lin 2004 and references therein) and gravitational disk instability (e.g. Boss 1997). The former involves the collisions and sticking of rock-ice planetesimals, which grow to Earth-size and beyond. Gas in the proto-planetary disk can also be gravitationally accreted (depending on location within the disk and its structure) to form giant planets. The predictions of this model are generally in qualitative agreement with current observations of planets within 3 - 5AU obtained from Doppler and transit searches. The second formation model—in which planets form through gravitational instabilities in the proto-planetary disk—is generally thought to operate at larger separations (i.e. > 5AU). Overall, a detailed understanding of planet evolution must also take into account the fact that planets do not necessarily form at the locations we observe them; they can migrate both within the proto-stellar disk where they form, and via dynamical interactions after the proto-stellar disk has dissipated.

Critical questions about planet formation that are currently unclear will likely remain the targets of active research for a decade or more. While it is probable that, as GMT goes into operation, improved versions of the core accretion and disk instability paradigms will still form the bases of our understanding of planet formation, outstanding questions will inevitably remain, such as:

 In what protostellar disk environments do the core accretion and/or disk fragmentation mechanisms dominate?  To what extent are the planetary properties we observe a result of formation processes, and to what extent are they a result of migration and dynamical evolution?  How do formation mechanisms impact the composition and structure of exoplanets? And how do they impact the composition of exoplanetary atmospheres?  What is the full range of planetary system multiplicities and structures (i.e. the range of system architectures) produced?  How common are Earth-analogue planets and how often are they as hospitable to life as Earth?

GMT will possess multiple capabilities critical for breakthrough exoplanetary science. Its huge aperture will allow us to acquire spectra of transiting planets 7.5 times faster than current 8m telescopes, enabling a new generation of spectroscopic studies of exoplanet atmospheres in the short windows allowed by primary- and secondary-transit durations. Its aperture will enable observations to lower photon noise limits, pushing Doppler measurements below the 10cm/sec precisions required to detect Earth-like planets in one- year orbits around the nearest stars. And GMT’s 24.5m effective diameter will allow it to achieve unprecedented spatial resolution. The potential for synergy when these capabilities are combined in one facility will be powerful. For example, while transit observations and spectroscopy with G-CLEF will identify and measure the masses of habitable planets, the GMT spectrographs will study their atmospheres, and near-IR imagers such as TIGER will be able to image them directly.

26 The Properties of Exoplanetary Systems | GMT Science Case

Together these capabilities will allow the GMT to lead us to a deeper and more profound understanding of the complex and chaotic mechanisms of planetary formation, and how they determine why planetary systems (including our own) look the way they do.

It is difficult to spell out in a single chapter the full range of the science that GMT will enable in this area. Indeed, in a field as young as exoplanetary science—one currently expanding in many different directions—predicting breakthroughs expected even in just a few years is problematic. Nonetheless, in the sections that follow we highlight three key science areas in which GMT can be expected to play a major role in contributing to our understanding of exoplanetary systems: atmospheres of gas giants, direct imaging, and habitable worlds.

3.2 Probing Exoplanet Atmospheres

Observing primary and secondary eclipses of transiting planets yields insights into their atmospheric compositions that cannot be obtained with other techniques. To date, observations of this nature have been performed using broad-band photometry and spectroscopy across the optical and infrared from both space and the ground.

During primary transit, when the planet crosses the stellar disc, spectroscopic observations of the depth of the transit can be used to determine the planet’s radius at a variety of wavelengths, and subsequently the properties of the transiting planet’s atmosphere (Figure 3.1). The key factor presently limiting such work is the difficulty in obtaining a high enough S/N ratio during short (typically ~1h) transit durations. GMT’s aperture (7.5 times larger than that of the VLT, from which the data in Figure 3.1 were obtained) will make it possible to achieve the required sensitivity during transits. GMT’s aperture will also make it possible to use this technique on smaller planets with thinner atmospheres, thus opening the door for probing the atmospheres of the potentially habitable planets that will be discovered by missions like TESS over the coming decade. Multi-object spectroscopy (enabling continuous monitoring of and correction for the Earth’s atmospheric transmission) is critical to making observations of this type from the ground. GMT will have the facility for doing so using GMACS & NIRMOS (at low spectral resolution) and GCLEF + MANIFEST (at high spectral resolution).

Figure 3.1 Primary Transit spectroscopy from the VLT has been used to probe the atmosphere of the 6MEarth planet GJ1214b (Bean et al. 2011).

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In secondary transit, when a planet passes behind the star, observations of the stellar light can be subtracted from the Planet+Star observations (obtained out of transit) revealing a planetary emission/reflection spectrum (e.g. Figure 3.2). The keys to these observations are high precision and high stability near-IR spectroscopy sufficient to measure contrast -3 levels of Fplanet/Fstar < 5x10 .

GMTNIRS and NIRMOS spectroscopy during secondary transits will provide unique new data on the atmospheres and compositions of eclipsing gas- and ice-giant planets. Line ratios and line shapes, particularly for molecular bands of CO and CH4, can reveal not only dayside temperatures, but also information about atmospheric pressures, temperatures, and chemical abundance profiles. Such data will yield valuable information about the formation locations of these planets, while also helping us to understand global circulation between the day and night sides in tidally locked systems. Indeed, the first high-resolution detection of the CO fundamental (Snellen et al. 2010a) suggests a significant day-night flow. The combination of greatly increased collecting area and GMTNIRS’ very large spectral coverage will permit further breakthroughs.

Figure 3.2 Secondary Transit observations can provide day-side photometry and spectra. In the case of WASP-12b, such data have suggested a C/O ratio > 1, and that this planet is unlikely to host a silicate core (Madhusudhan et al. 2010).

3.3 Imaging Exoplanetary Systems

Exoplanet direct imaging will provide otherwise unobtainable data on the formation, evolution, and multiplicity of exoplanets in wide orbits (i.e. 5 - 50AU), critically complementing Doppler and transit observations, which are inherently more sensitive at smaller orbital separations.

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Targets for direct imaging fall into a few distinct classes:

• Planets still embedded in their parent disks (age = 1 - 10Myr, at 30 - 150pc). These are discussed in detail in Chapter 2; • Young (0.1 - 1Gyr), nearby (3 - 50pc) gas-giant planets, which are intrinsically bright in the near-infrared due to their on-going gravitational contraction and, • Older (> 1Gyr) planets detectable via their thermal infrared emission or reflected light.

The GMT will provide high contrast, high resolution imaging capabilities in the near and mid-infrared enabling the detection of exoplanets in each of these categories.

Figure 3.3 The multiple planet system orbiting HR8799 has been resolved by multiple ground-based AO systems, including this observation at 3.8μm by Hinz et al. (2010). Direct imaging observations like this are a powerful and immediate probe of the multiplicity and system architectures of the outer regions of exoplanetary systems.

One of the technical goals of the GMT is to detect objects more than one million times fainter than the host star at angular separations corresponding to 1.5 λ/D to 20 λ/D as Figure 3.3, an image of the exoplanet system HR8799 illustrates. Figure 3.4 shows one approach to meeting this goal, in which phase-apodization coronagraphy is used to redistribute diffracted light to one side of the star, carving out a deep, high-contrast swathe near the star. GMT’s large primary mirror segments enable a variety of innovative schemes for diffraction suppression in this vein.

Observing the properties of ice giants in particular (e.g. those beyond the snow line) is crucial to our understanding of planet formation, since these planets undergo little dynamical interaction and are likely to be observed near where they formed. Moreover, these observations are a powerful test of exoplanet formation models, since both the core accretion and gravitational instability paradigms make specific predictions for the nature

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of exoplanetary systems in their outer regions. Direct imaging will also permit the detailed study of exoplanetary atmospheres, thus constraining temperatures, sizes, masses, and the compositions of exoplanets. The GMT’s capabilities in planetary discovery and subsequent detailed analysis will enable testing exoplanet formation and evolution models.

Figure 3.4 Reflected light Inner-Working-Angle-Contrast Diagram for GMT (left) and a simulated image (right) showing the dark region of extremely high contrast sensitivity “dug out” by the phase apodization coronagraphy technique.

3.3.1 Young Gas-Giant Planets

The L’-band is likely to be the most sensitive window for imaging giant planets. In this band, GMT will have a resolution limit (2 λ/D) corresponding to 46mas, and in a 1-hour observation will be able to reach a sensitivity limit of 20th magnitude. In Table 3.1 we list some representative limiting masses for GMT observations of exoplanets with ages ranging from 10Myr to 1Gyr (based on the COND03 models of Baraffe et al. 2003). In Figure 3.5 we show simulated GMT observation of planets in the β Pic system (already known to host one 10MJup planet). A design reference survey to L’ = 20 for some 50 nearby stars with a median age of 0.5Gyr would be expected to detect some 15 - 20 new exoplanets with separations of 1 - 20AU. Follow-up photometry and low resolution spectroscopy at wavelengths between 1 and 13μm will be particularly critical for determining atmospheric and surface properties of these planets.

Table 3.1 Detectability of planets via direct imaging

Separation Distance (pc) Age Mass Limit (AU)

50 10 Myr 2.3 <0.5 MJ

150 10 Myr 7.0 0.5 MJ

20 0.3 Gyr 1.0 1 MJ

10 1 Gyr 0.5 1.5 MJ

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Figure 3.5 Simulated GMT observations of the β Pic system (compared to current state-of-the-art VLT L’ data). GMT will not only trivially detect the known 10MJup planet, but will be able to detect planets of Saturn mass beyond 3AU in this system.

3.3.2 Spectroscopy in the Near-Infrared

The massive exoplanets detected in recent years at wide orbital separations (Marois et al. 2008, 2010; Kalas et al. 2008) present something of a mystery—it is unclear whether these massive planets form at their observed locations, or whether (like Neptune) they have migrated outwards to their present orbits. Clever use of current-generation AO integral-field spectrographs has enabled ground-based infrared observations at moderate spectral resolution (Patience et al. 2010, Barman et al. 2011). GMTNIRS (used in combination with the GMT facility AO system) will be able to observe the brightest of the distant planets currently known (and by extension large samples of the planets that will be known in the next decade) at ~100 times the spectral resolution of current work. These observations will give a far more detailed physical picture of these objects enabling us to use data on their composition and structure to determine where they formed, as opposed to where they are observed.

3.3.3 Thermal Imaging of Older and Smaller Planets

Planets that are either older or less massive will have temperatures set by their parent stars separations. Radial velocity searches have already uncovered a rich sample of these objects (currently, more than 70 such planets with angular separations of > 62mas are known). The GMT’s resolution will allow detection or direct imaging of a significant subset of these known objects (Table 3.2) and a much larger sample is likely to be known by the time the GMT begins operations.

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Table 3.2 Potential Doppler planets (currently known) detectable at L’ in their self-luminosity with GMT+TIGER

Planet Separation L’ Planet Separation L’  Eri b 1.06” 21.3 GJ 676 A b 0.11” 21.6 HIP 70849 0.42” 20.1 HD 8673 b 0.08” 20.7 HD 87883 0.20” 18.9 HD 106252 b 0.07” 21.7 υ And d 0.19” 18.5 HD 111232 b 0.069” 21.6 HIP 5158 c 0.17” 21.2 υ And c 0.064” 18.5 HD 39091 b 0.16” 19.4 HD 60532 0.061” 20.9

Another exciting prospect for GMT lies in its potential to detect thermal emissions from low-mass (and potentially rocky) planets that Doppler and transit surveys suggest are relatively common. Doppler search data sets in particular are indicating that 20 - 30% of Sun-like stars host planets of less than 10MEarth at periods of less than 50 days (Howard et al. 2010, Wittenmyer et al. 2011b). Initial analyses of early Kepler transit data are confirming these data by showing that 20 - 30% of stars host planets of 2REarth or smaller (Howard et al. 2011) at periods of less than 50d. Over the next few years these data will probe the frequency with which Earth-sized planets orbit at periods of 0.5 - 1 year, and the trends available in extant data suggest that up to 100% of stars may host small planets within 1AU.

It is likely that by the time GMT goes into operation, nearby stars hosting such planets will have been identified by either transit searches (e.g. TESS) or on-going Doppler survey programs, providing a wealth of new targets for GMT direct imaging studies. The nearest stars (i.e. d < 10pc) will be prime targets for GMT direct imaging of these relatively common rocky planets in the L’ (3.8μm) or N (10μm) bands. The expected angular separation of a rocky planet from its parent star with a peak flux near 10μm is simply a function of the apparent magnitude of the star, so the most attractive stars to search are also the brightest ones in the sky.

Table 3.3 lists (for 9 of the nearest stars) the spatial separation at which a planet would maintain a “Warm Earth” temperature of 280K, suitable for N-band detection. Planets at these separations around these stars would lie within their system’s habitable zone and be outside the GMT’s 1.5λ/D inner working angle threshold. The N-band brightness of a 2REarth planet at 5pc is approximately 9μJy, suggesting that super-Earths would be well above the expected noise level in a several hour observation.

Of course, rocky planets may exist in a range of orbits around their parent stars. While planets at larger separations would be cooler and less detectable, planets closer to their stars will have equilibrium temperatures correspondingly higher, and would be even more amenable to study. An Earth-size planet at an equilibrium temperature of T = 600K would be roughly four times closer than a true Earth analog (at 280K) and would have a peak flux in the L-band atmospheric window. If we assume blackbody emission of the planet at its equilibrium temperature, it would have an absolute flux of 2μJy, which

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compares favorably with the photometric limit of 0.5μJy, expected for a GMT limited by sky and telescope background in a 1-hour observation. While the approximation of a 600K blackbody for a hot Earth is almost certainly inaccurate, spectral models for hot planets with a range of atmospheric compositions have been examined by Miller-Ricci et al. (2009). These models suggest that because the L'-band is a generally transparent window, the detection of “hot Earths” at L' is feasible with the GMT.

Table 3.3 Potential targets for rocky planet detection

“Warm Earth” “Hot Earth” Object D (pc) 280K planets 600K planets

 Cen A 1.35 770 mas 170 mas  Cen B 1.35 450 mas 100 mas  CMa 2.64 2400 mas 520 mas  Eri 3.22 170 mas 40 mas  CMi 3.5 490 mas 110 mas  Cet 3.65 210 mas 50 mas  Aql 5.14 650 mas 140 mas  Hyi 7.47 280 mas 60 mas  PsA 7.69 640 mas 140 mas

The factors limiting the detectability of “warm” versus “hot” Earths are different. Photometric sensitivity in the N-band limits the number of stars around which a warm rocky planet can be detected. For L' “hot Earth” studies, the available photometric sensitivity will, in principle, allow detection of even sub-Earth mass planets, but only for planets outside the ~1.5 λ/D separations shown in Table 3.3. The science that would come from the observation and subsequent detailed study of these classes of planets would be truly groundbreaking.

3.4 Probing the Nearest Habitable Planets Using Doppler Spectroscopy

3.4.1 Earth-analogues Orbiting Sun-type Stars

Doppler velocity measurements detected the very first known exoplanets, and for the first fifteen years of the current “Exoplanetary Era” they have dominated the field. In recent years, transit and direct imaging observations have begun playing increasingly important roles, and the Kepler mission is particularly poised to revolutionize our understanding of the statistical distributions of exoplanet frequency for small planets. Nonetheless, Doppler and transit observations will remain critically complementary in the decade ahead, since only the combination of these two techniques can reveal both exoplanet mass and exoplanet size. This combination delivers exoplanet bulk density, which is a critical parameter in determining whether the potentially habitable low-mass planets we find orbiting other stars are rocky, small ice-giants, or water worlds.

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Detecting and studying planets with masses below 5MEarth will be one of the highest profile objectives for astronomy over the next ten years. The recent U.S. Decadal Report New Worlds, New Horizons in Astronomy and Astrophysics ranks this objective as one of astronomy’s highest priorities, with an ultimate goal being to find evidence for life outside the Solar System. Such an objective is in all respects remarkable, particularly when one considers that the current generation of astronomers may see it achieved within their lifetimes.

GMT will play a fundamental role in this endeavor. Its massive aperture combined with the high-precision GCLEF spectrograph will enable it to carry out a census of the population of Earth-like planets orbiting the nearest Sun-like stars. Such a census requires sub-10cm/sec Doppler velocity precisions at multiple epochs over a planetary orbital period. Recent Doppler detections by the HARPS spectrograph indicate that these extraordinary levels of Doppler precision are achievable. Indeed, the ability to detect planets down to a few Earth-masses at periods of up to 90d has been demonstrated by obtaining multiple observations averaged over a single night, and 40 - 50 epochs per year over several years (Pepe et al. 2011). Moreover, when those detected planets are removed from the data sequence, and the resulting residuals binned over periods of 40 - 50d, the resulting data set displays residual velocity dispersions that drop below 20cm/sec (Figure 3.6).

This result shows that there are stars in the solar neighborhood with the intrinsic Doppler stability (at periods of interest) to detect Earth-like planets, as long as sufficient photons can be detected, and astrophysical sources of noise can be averaged over. Observations that achieve these goals can potentially detect the much sought-after habitable Earth- analogue planets in ~1AU orbits.

Figure 3.6 Binned residuals to the HARPS Doppler data for the triple planet system orbiting HD20794 (Pepe et al. 2011). When the planets detected are removed, and the data binned to average over stellar noise at periods up to ~40d, the resulting Doppler dispersion drops to below 20cm/sec.

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Obtaining these data for a meaningful sample will require the combination of the GMT’s massive aperture and GCLEF’s extraordinary stability and precision. The HARPS data described above for HD20794 was collected by integrating nine 4m-telescope nights over 3 years, all dedicated to this one target, delivering a ‘per night’ dispersion of 82cm/s. Dropping this dispersion down to 10cm/sec will require averaging over some 70 epochs to acquire the extra factor of 8.2 in signal-to-noise required. This detection, then, is completely unfeasible on a 4m or 8m telescope. The GMT’s aperture, however, will collect photons 37 times faster than HARPS’ 3.6m telescope, making it possible to study a sample of ten Earth-analogue host stars in less than 36 nights per year over the first 5 years of GMT’s operations. It should be noted that this represents the most conservative possible calculation, and one based only on presently achieved Doppler precision.

The history of this field over the last 15 years is marked by relentless advances in precision, a trend that shows no sign of abating. Such advances will permit Doppler studies to make even more efficient use of the GMT. Even with the most conservative expectations GMT will deliver groundbreaking results in reasonable amounts of observing time.

3.4.2 Habitable planets orbiting M-dwarf Stars

A complementary strategy to searching for habitable planets orbiting stars like the Sun is to search for low-mass planets orbiting stars much smaller than the Sun—M-dwarfs. Habitable planets in these systems will lie at smaller orbital separations (or equivalently shorter orbital periods) making them potentially much easier to detect.

Figure 3.7 The theoretical habitable zone (shaded region) as a function of stellar mass. Planets inward of the dashed line become tidally locked in less than 1 Gyr. Venus, Earth, and Mars are indicated for the Sun. The orbital positions and relative sizes of the first three planets discovered orbiting the M dwarf GJ 581 (Udry et al. 2007) are also shown. The corresponding orbital periods are between approximately 4.5 and 70d (Selsis et al. 2007).

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GMT will be able to play a critical high-profile role in this endeavor as well. Table 3.4 compares the orbital periods and Doppler amplitudes that result for 1MEarth planets in orbits similar to those of Venus, Earth and Mars (which approximately span the potentially habitable zone within the Solar System) around a 1MSun star, with the same numbers for a 0.3MSun M-dwarf. The detection of habitable M-dwarf planets will require precisions at the 40cm/sec level. While these are substantially larger than limits for Sun- like host stars, they remain challenging because M-dwarfs are faint in the optical. With its large aperture, the GMT will reduce the time required to make these observations from seven hours (for an 8m telescope and a V = 16 M-dwarf) to less than one hour. The combination of GMT’s huge aperture, together with a class-leading high-precision red- sensitive spectrograph like GCLEF, would enable GMT to deliver paradigm changing science within its first 5 years of operation.

Table 3.4 Doppler amplitudes for Earth analogue planets

Star Planet a (AU) P (d) K (cm/s)

1.0 M Venus 0.72 225 15  1.0 M Earth 1.00 365 11  1.0 M Mars 1.52 687 9 

0.3M “Venus” 0.1 22 50  0.3M “Earth” 0.13 31 45  0.3M “Mars” 0.2 60 37 

3.5 Summary

GMT is set to make groundbreaking discoveries and undertake fundamental science across the field of exoplanetary science. But the scientific programs highlighted above (planetary transit spectroscopy; direct exoplanetary imaging; and habitable world detection and study) by no means represent the sum total of its capabilities. GMT will have the spectroscopic capabilities to explore potential elemental anomalies in exoplanet host stars (e.g. Melendez et al. 2009); it will have the aperture and speed to enable the measurement of exoplanet spin-orbit alignments (e.g. Winn et al. 2010) and to measure the multiplicities and architectures of systems previously discovered using transit and Doppler techniques; it will have the sensitivity to probe the variability of atmospheric structures of exoplanets and brown dwarfs as they rotate; and it will have the power to undertake a plethora of research programs as yet unanticipated in this, the youngest of all the branches of astrophysics.

36 The Properties of Exoplanetary Systems | GMT Science Case

References

Bean, J. et al. 2011, ApJ, submitted, arXiv:1109.0582 Baraffe, I. et al. 2003, A&A, 402, 701 Borucki, W. et al. 2011, ApJ, 736, 19 Boss, A. 1997, ApJ, 661, L73 Barman, T., S. et al. 2011, ApJ, 733, 65 Hinz, P. et al. 2010, ApJ, 716, 417 Howard, A. et al. 2010, Science, 330, 653 Howard, A. et al. 2011, ApJ, submitted (arXiv:1103.2541) Ida, S., & Lin, D. N. C. 2004, ApJ, 604, 388 Kalas, P. et al. 2008, Science, 322, 1345 Madhusudhan, N. et al. 2010, Nature, 469, 64 Marois C. et al. 2008, Science, 322, 1348 Marois, C. et al. 2010, Nature, 468, 1080 Melendez, J. et al. 2009, ApJ, 705, L66 Miller-Ricci, E. et al. 2009, ApJ, 690, 1056 Patience, J. et al. 2010, A&A, 517, 76 Pepe, F. et al. 2011, A&A, 534, A58 Snellen, I.A.G. et al. 2010, Nature, 465, 1049 Tinetti, G. et al. 2006, Astrobiology, 6, 881 Winn, J. 2010, in “Exoplanets,” ed. S. Seager, University of Arizona Press (Tucson, AZ) Wittenmyer R. et al. 2011a, ApJ, 727, 102 Wittenmyer R. et al. 2011b, ApJ, 738, 81

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4 Stellar Populations and Chemical Evolution Anna Frebel and Gary Da Costa with Warren Brown, Joe Hora, Duncan Forbes and Andrew McWilliam

4.1 Introduction

The current decade promises to be a “decade of surveys.” With planned or ongoing photometric and spectroscopic surveys such as SDSS-III (BOSS, APOGEE, SEGUE-2, MARVELS), Pan-STARRS, DES, SkyMapper, VISTA, VST, LAMOST, HERMES, Gaia and LSST, all areas of astronomy will experience an explosion of new data and discoveries. While massive amounts of survey data can answer many scientific questions, the need to study individual stars or relatively small numbers of objects in much greater detail will remain a central focus of astronomy.

Such a focused approach is particularly valuable for studying our own Galaxy since, to this day, many key properties of the Milky Way are not well understood. For example, how did the different stellar populations (e.g., thin and thick disk) assemble? What was the Galaxy’s chemical evolution? Are the oldest and most metal-poor stars mostly located in the bulge as simulations suggest? Is the halo made from shredded, small dwarf galaxies? These questions can only be addressed with a large telescope like GMT using spectroscopic studies of stars in the Milky Way and in the Local Group dwarf galaxies, and by imaging resolved stellar populations in more distant galaxies.

Figure 4.1 Field of Streams: A map of stars in the outer regions of the Milky Way Galaxy, shown in a Mercator-like projection. Color indicates the distance of the stars, while intensity indicates the density of stars on the sky. Structures visible in this map include streams of stars torn from the Sagittarius dwarf galaxy, a smaller “orphan” stream crossing the Sagittarius stream, the “Monoceros Ring” that encircles the Milky Way disk, trails of stars being stripped from the globular cluster Palomar 5, and excesses of stars found towards the constellations Virgo and Hercules. Circles enclose Milky Way companions, newly discovered by the SDSS; two of these are faint globular star clusters, while the others are faint dwarf galaxies. From Belokurov et al. (2006) and the Sloan Digital Sky Survey.

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Figure 4.1 shows an example of current survey data outcomes: the Field of Streams. This image clearly shows the star streams and stellar clouds in the Milky Way left behind from the accretion and disruption of satellite dwarf galaxies. Data of this nature have already provided much information on the formation of the Galaxy’s halo, but exploiting the full potential of information contained in survey data requires extensive kinematic and spectroscopic follow-up of individual stellar objects, a program that the GMT is ideally suited to carry forward.

Baade’s early recognition of distinct stellar populations has provided an enduring framework for studies of galaxy formation and evolution, stellar chemistry, and stellar dynamics. Today, any viable model for the formation of the Milky Way must address the origin of thin disk, thick disk, bulge and halo populations, and the diversity of their dynamical, chronometric, and chemical properties. Our understanding of stellar populations in external galaxies is still quite limited and is based largely on integrated light spectra. With its southern location and well-suited instruments, the GMT will open a number of avenues for detailed studies of stellar populations in the outer halo, the Local Group and beyond, in concert with SkyMapper, VISTA, VST, LSST, DES and other southern surveys.

More generally, GMT will greatly enhance stellar science related to spectroscopy and photometry. With regard to spectroscopy, GMT will facilitate the acquisition of high- resolution spectra with extremely high S/N of brighter targets beyond what is possible with current 8m telescope/spectrograph combinations, as well as reach out farther into the Universe to study fainter and currently inaccessible targets. Achieving many of these science goals will in part rely on GMT’s multi-object spectroscopic capabilities. In the realm of photometry, studying more distant objects than are currently accessible is one of GMT’s primary goals. With the high spatial resolution provided by laser tomography AO, it will be possible to carry-out near-IR photometry of resolved AGB stars in systems out to the Coma Cluster thereby constraining their stellar populations.

Stellar population science as a whole aligns extremely well with the top priorities of the Astro2010 Decadal Survey, New Worlds, New Horizons. Studies of low metallicity objects in the Galactic halo and in dwarf galaxies, and studies of high-redshift galaxies at the “Cosmic Dawn”, are major science goals for the current decade. A suitably instrumented GMT will be able to address these top-priority topics.

The GMT will provide a new window to the Universe promising to produce new and exciting discoveries, many of which cannot be predicted at present. That being said, GMT will likely advance our understanding of the chemical evolution of the Universe from early stochastic beginnings to those of the Milky Way, the Local Group and other more distant galaxies. It will follow the drivers of star formation and evolution over the history of the Universe all the way to the details of the formation of solar systems and habitable planets.

In the sections below we highlight a few of the science topics to be explored with GMT in the general area of stellar populations and chemical evolution.

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4.2 Stellar Archaeology

4.2.1 Characterizing the Most Metal-poor Stars

Extremely metal-poor stars ([Fe/H] < –3.0) predate the main halo population, possibly from a pre-Galactic era. Some stars are so metal-poor and have such unusual compositions that they are thought to reflect the composition of individual supernova events after the Big Bang (e.g., McWilliam 1995; Beers & Christlieb 2005; Frebel 2010). These objects are profoundly important for understanding the very first chemical enrichment in the Universe, and provide important constraints on the nature of Population III stars, their IMF, and the yields of the first massive core-collapse supernovae. Unfortunately, since these stars are so metal-poor (the most metal-poor stars currently known have [Fe/H] ~ –5.5) the strongest absorption lines of the most important elements will be too weak to measure in normal high dispersion spectra with S/N ~ 100 (Frebel et al. 2005, Aoki et al. 2006).

Figure 4.2 shows the spectrum of the most metal-poor star currently known: the bright (V = 13.5) warm subgiant HE1327-2326 which has Teff ~6200K and [Fe/H] = -5.4. Also shown, for comparison, is the spectrum of extremely metal-poor turnoff star, G64-12 for which [Fe/H] = -3.2. The region around the strongest Fe line is displayed and its equivalent width is only ~7mÅ in the HE1327-2326 spectrum. If this object had been somewhat hotter or somewhat more iron deficient, no Fe line could have been measured with current facilities. Given the odd chemical abundance pattern of extremely high [C, N, O/Fe] element ratios, and elevated [alpha/Fe] (found in both of the currently known [Fe/H] < -5.0 stars), accurate measurements of the Fe abundance from several extremely weak Fe lines are crucial.

Figure 4.2 Spectral region around the strongest Fe line in the optical wavelength regime at 3860Å. It illustrates the effect of metallicity on line strengths for the Sun and three additional stars with decreasing metallicity. Compared to G64-12, this line in HE 1327-2326, with [Fe/H] = -5.4, is hardly detectable. All other lines are indistinguishable from the noise. Figure from Frebel & Norris (2012).

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Of course, cooler giants will have intrinsically stronger lines, making Fe line detection somewhat easier, at least down to [Fe/H] ~ -6 to -7. However, given the paucity of these stars coupled with their extreme importance for the study of the early Universe, all [Fe/H] < -5.0 stars will need to be spectroscopically studied in great detail to constrain the conditions shortly after the Big Bang, in line with the recommendations of the Astro2010 Panel on Stars and Stellar Evolution.

The present decade will uncover new, extremely metal-poor stars through the next generation of photometric and spectroscopic surveys, thus advancing the field of stellar archaeology. The SkyMapper Southern Sky Survey (Keller et al. 2007), for example, expects to provide a 100-fold increase in the numbers of [Fe/H] < -3 stars, as well as up to 50 objects with [Fe/H] < -5.0. Most of these stars, however, will be too faint for high resolution spectroscopy with current facilities. Only with the light-collecting power of an ELT such as the GMT will it be possible to study these fossil stars with the required precision to generate a much-improved understanding of the early Universe and the processes governing nucleosynthesis.

GMT could make major contributions to stellar archaeology through a two-component observing program. A discovery mode (R ~ 20,000, S/N ~ 30 at 4000Å) would be used to confirm the metallicity of pre-selected targets from large surveys and to quickly identify any unusual abundance characteristics (e.g. ultra-metal-poor, or r-process enhancement). Important targets would then be observed with higher resolution (R ~ 40,000) and high S/N (~100 - 150 or more, depending on the nature and metallicity of the object) to acquire data required for a detailed chemical abundance analysis.

In a modest allocation of nights over 2 - 3 years one could survey several hundred stars to V ~ 18, selecting a sample of a few tens of stars for detailed abundance work. Over the course of a few years, a modest sized group working within the GMT community could then extract a great deal of value-added science from the large-scale photometric and spectroscopic survey programs planned for the coming decade. In particular, such studies would permit a much more accurate definition of the metallicity distribution function (MDF) at the lowest metallicities. The MDF is currently poorly defined as the number of known extremely metal-poor stars is small: for example, at the present time there are only a handful of stars known with [Fe/H] < -4.0. Characterizing the extremely metal-poor tail of the MDF is, however, pivotal for constraining both models of the formation of the first stars and the role of processes such as SNe feedback in the earliest stages of galaxy formation (e.g., Salvadori et al. 2007).

Such studies will also play a vital part in defining the role of carbon abundances at the lowest metallicities. Approximately 20% of stars with [Fe/H] ~ -2 are significantly enhanced (by a factor of ~10 or more) in carbon relative to the scaled solar value, and this percentage rises to ~100% at the lowest abundances. Carbon is probably the first element to enter the ISM and is therefore likely to be a major driver of dust formation at the earliest epochs. Yet the origin of the substantial carbon enhancements at the lowest metallicities is not well understood, as there are potentially a number of production mechanisms involving stars of different mass. Nevertheless, with detailed abundance

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studies of a large sample of extremely metal-poor stars, we can expect to untangle the role of carbon enhancements and thus advance our understanding of the physical processes that govern star formation at the earliest times.

4.2.2 Age Dating the Oldest Stars

A classical test of Cosmology lies in comparing the ages of the oldest stars in the Galaxy with the age of the Universe as inferred from cosmological parameters. The ages of the oldest stars can be directly determined by measuring the abundances of long-lived radioactive isotopes such as 232Th (Thorium, half-life 14Gyr) and 238U (Uranium, half-life 4.5Gyr). Specifically, the age of a star can be derived by comparing the abundances of Th and/or U with those of stable r-process nuclei, such as Europium (Eu). While Th is often detectable in metal-poor stars that are enhanced with r-process elements, determining U abundances is particularly challenging because the only U line in the optical and near-IR spectral region with sufficient strength to permit abundance determinations is extremely weak and lies in the UV-blue spectral region (~3860Å) where there are many other contaminating lines from more abundant species.

Figure 4.3 Spectral region around the only available uranium line in HE1523-0901, the r-process enhanced star with the strongest overabundance in heavy neutron-capture elements in comparison to CS31082-001, the only other star with a U detection (but 200K cooler than HE1523-0901, resulting in a significantly weaker U line). The solid blue line shows the computed spectrum for the case of no Uranium present in the stellar atmosphere, while the dotted blue line shows the strength of the line if no Uranium had decayed. A larger region is shown on the left, with a zoom-in version of the right. Figure from Frebel et al. (2007).

Currently, 238U has only been detected in three stars, and one of these detections is tentative. For HE 1523-0901 (V = 11.1) and CS 31082−001 (V = 11.3), high-resolution spectra with R ~ 80,000 and S/N ~ 350 - 500 at 4000Å were required to successfully measure the optical U line from which the abundance was deduced (Frebel et al. 2007). The spectral region around the U line in these stars is shown in Figure 4.3. HE1523-0901 was determined to have an age of 13.2 Gyr by averaging the results of several nucleo- chronometers involving combinations of the elements Eu, Os, Ir, Th and U (Frebel et al.

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2007), while for CS 31082-001, the U/Th chronometer yielded an age of ~14Gyr (Cayrel et al. 2001). These ages provide a lower limit to the age of the Galaxy and hence, to the Universe. They are consistent with the age of 13.75Gyr derived from cosmological parameters measured in the WMAP experiment (Larson et al. 2011).

Uncertainties in stellar age determinations are significant, ranging from ~2 to ~5Gyr, and are driven primarily by the uncertainty in the U abundance determination. However, with more determinations from a larger sample of stars, the statistical uncertainty can be reduced leaving systematic uncertainties, such as those in the initial production ratios (e.g. for U/Th) as the dominant error source. Current estimates place the initial production ratio uncertainties as contributing an age uncertainty of ~1.6Gyr in the best case (Frebel et al. 2007).

The stars most suitable for direct age determinations are cool metal-poor giants that exhibit strong overabundances of the r-process elements. Unfortunately, such stars are often also very carbon-rich and the 238U line is blended and not measureable. For example, in the extreme r-process star CS22892-052, only the Th/Eu ratio could be employed yielding an age of ~14Gyr (Sneden et al. 2003). Compared to Th/Eu, the U/Th ratio is much more robust, however, with respect to uncertainties in the theoretically derived production ratio, as a result of the similar atomic masses of Th and U (Schatz et al. 2002). Hence, old metal-poor stars displaying both Th and U are the most valuable for nucleochronometric age determinations. Candidates for further study will come from large area surveys of very metal-poor stars such as SEGUE, SkyMapper and HERMES.

With GMT’s optical high resolution spectrograph it will be possible to obtain spectra of stars suitable for nucleo-chronometric age determinations with S/N of 500 or more in just a few hours. This S/N and a resolution of R > 50,000 are required to detect and adequately measure the weak U line. Spectra of this quality will also enable the detection of weak lines of a number of other rarely studied elements permitting a full chemical characterization of the r-process element enhanced metal-poor stars. For example, Lead (Pb) measurements in these stars (Pb being the decay product of Th and U) are even more challenging (S/N > 500 required), but they will provide the ultimate empirical constraint for r-process modeling and thus nuclear astrophysics. Ultimately, a large stellar database with measurements of the heaviest chemical elements will not only provide a constraint on cosmological models but will also further our understanding of the production of the heavy elements in the Universe.

In summary, detailed abundance measurements of the most metal-poor stars and nucleochronometric studies of r-process enhanced stars will be critical for constraining nuclear reaction processes and supernova yields, as well as increasing our general understanding of the earliest phases of chemical evolution in the Galaxy and the Universe.

4.3 Abundances in Dwarf Galaxy Stars

Red giant stars are particularly useful for measuring the history of chemical evolution in

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nearby galaxies. Their high luminosity makes them accessible at great distances, their low temperatures result in the presence of lines from many elements and molecules, and their progenitors have ages up to ~13 billion years. As such, the chemical composition of red giant stars provides a fossil record of the history of galaxy evolution.

The GMT will routinely measure chemical abundances in RGB stars in Local Group galaxies visible from the southern hemisphere. This includes nearby galaxies such as the irregular galaxies LMC and SMC, and a host of dwarf spheroidal (e.g., Sculptor, Carina, Fornax) and dwarf irregular galaxies (e.g., NGC 6822, IC 1613, WLM). The recently discovered ultra-faint dwarf galaxies, with their close connection to the building blocks of the Galaxy, are also prime targets. While they are closer, these objects contain relatively few stars, so observations somewhat down the RGB are crucial in order to ensure that sufficient numbers of stars per galaxy can be studied. This topic is examined in more detail in Chapter 5 (Galaxy Assembly).

Acquiring high resolution spectra of RGB stars in more distant Local Group galaxies, such as NGC 6822 at 495kpc, IC 1613 at 725kpc, Tucana at 890kpc and WLM at 925kpc would require considerable effort: up to 35 one-hour exposures for S/N ~ 30. But this would be worthwhile if the spectrograph has multi-object capabilities; one might obtain S/N ~ 30 spectra for several hundred RGB stars in one of these galaxies within a few nights. An example of what such a spectrum would reveal is shown in Figure 4.4. It will be particularly interesting to compare the chemical enrichment histories of dwarf irregular and dwarf spheroidal galaxies. Such a comparison will help elucidate the evolutionary connection between these two classes of dwarf galaxies.

Figure 4.4 Simulated GMT typical high resolution spectrum anticipated for individual RGB stars in the halos of Local Group galaxies (S/N ~ 30, R ~ 25,000). For RGB stars at a typical distance of ~800kpc this would require approximately 35 one-hour exposures. The same quality spectrum could

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be obtained for RGB stars in galaxies at 2Mpc by combining 40 multi-object spectra, each comprised of 24 one-hour exposures. Figure from A. McWilliam, private communication.

4.4 Probing the Origin of the Milky Way’s Halo

Understanding the formation and evolution of the Milky Way halo was one of the great challenges of late 20th century astrophysics. Efforts to reconcile the kinematics and chemical composition of halo, bulge and disk stars led to one of the great debates in the study of galaxies: the monolithic collapse model, embodied in the Eggen, Lyden-Bell and Sandage (1962) paper, juxtaposed against the satellite accretion model put forward by Searle and Zinn (1978). Today, the Milky Way halo provides a unique laboratory for comparing galaxy formation models and the ΛCDM hierarchical merging paradigm with small-scale structure and in-situ velocity determinations. The compositions, spatial distribution and motions of stars in the distant Milky Way halo can simultaneously address a number of important scientific issues including:

1) Probing the mass profile of the Milky Way to ~50kpc or more via measurements of the velocity dispersion profile, and

2) Quantifying the role of satellite accretion by identifying star streams in position and velocity space, and measuring their orbits and chemical compositions.

While undoubtedly contributing to advances in both of these areas, the GMT will make significant contributions to quantifying the role of satellite accretion in the formation of the Milky Way halo. This will come about through GMT’s ability to conduct detailed abundance analyses of distant Milky Way halo stars. Typical targets will fall into one of two categories: stars identified through photometric and kinematic studies as being members of distant stellar streams, or members of distant ultra-faint dwarf galaxy satellites of the Milky Way. The latter systems are discussed in more detail in the following chapter but they are believed to be remnants of a larger population of similar systems whose disruption contributed stars to the stellar population of the halo. This hypothesis, however, is in need of observational confirmation: do the abundances and element-to-iron abundance ratios of stars in the ultra-faint satellites agree with those of the halo field population? Current data (e.g., Norris et al., 2010) suggest the answer may be ‘yes’, but larger samples of stars in the more distant systems are needed for confirmation. Such studies, however, require an ELT equipped with an efficient high dispersion spectrograph such as GMT’s G-CLEF.

Equally significant is the recognition (e.g., Norris et al., 2008) that, given the low stellar masses and mean abundances, the chemical evolution of the ultra-faint dwarf satellites is strongly influenced by stochastic effects, as only a relatively small number of supernovae are required to enrich the primordial gas to the observed abundance level. This suggests that in ultra-faint dwarf satellites, we will detect star-to-star differences in element abundance ratios that reflect individual supernovae events. These can then be used to constrain the masses of the supernova progenitor stars with the outcomes being generally applicable to star formation processes at the earliest times.

Further, as noted in the Introduction, recent photometric surveys of large areas of sky GMT Science Case | Stellar Populations and Chemical Evolution 45

have identified numerous stellar streams in the Galactic halo (see Figure 4.1), with the highest contrast streams occurring in the outermost parts of the halo. Follow-up of candidate stream members with intermediate resolution multi-object spectroscopy enables both the confirmation of stream membership and the determination of stream kinematics, allowing the orbit of the progenitor to be estimated (e.g., Odenkirchen et al. 2009). Detailed abundance studies of stream stars, made possible by the GMT and its high dispersion spectrograph, then have the potential to characterize the nature of the stream progenitor. For example, if the stream stars have a small or zero dispersion in heavy element abundances and halo-like element-to-iron abundance ratios, then the progenitor was most likely a globular cluster. In the likely more common case of a disrupted dwarf galaxy, the mean abundance of the stream stars constrains the luminosity of the progenitor, as luminosity and mean abundance are correlated for dwarf galaxies (e.g., Kirby et al. 2010), while the alpha element (e.g. Ca) to iron abundance ratios yield information on the star formation history of the system.

With studies of a number of streams, particularly those to be discovered in southern hemisphere sky surveys such as SkyMapper, VISTA and LSST, estimates of the luminosity function of the disrupted systems can be made and compared with those for the present-day dwarf galaxy satellites that are survivors of the merger/accretion process.

4.5 Globular Clusters in Local Group Galaxies and Beyond

With its large collecting area and high angular resolution in the diffraction-limited mode, GMT will be able to uniquely contribute to our understanding of dense stellar systems. Globular clusters in Local Group galaxies and other close neighbors can be studied in great detail with the GMT, while integrated light studies can probe the dynamics and abundances in globular cluster systems associated with large galaxies and high stellar densities in the Virgo and Coma Clusters.

The case for photometric studies of resolved stellar populations with ELTs has already been made, for example, by the GSMT science working group2. Figure 4.5 shows a simple graphical simulation of diffraction-limited images of a globular cluster with a core radius of 3pc at the distance of Centaurus A (NGC 5128). The three panels show images as imaged with Hubble, a diffraction-limited 8m, and the GMT using the LTAO adaptive optics mode. The reduction in crowding noise is dramatic and illustrates one of the strengths of the ELTs: their ability to access crowded environments beyond the reach of Hubble, current ground-based telescopes, and the James Webb Space Telescope.

Color-magnitude diagram studies based on near-IR imaging with LTAO have the potential to provide much new information on the ages (via the luminosities of thermally pulsing AGB stars) and metallicities (via red giant branch colors) for both the star clusters and the field-star populations in many galaxies within the local Universe (D < 10 - 15Mpc). Due to the large diversity of galaxy types in this volume compared to the Local

2 “Frontier Science Enabled by a Giant Segmented Mirror Telescope”, Section 5. See http://www.gsmt.noao.edu/gsmt_swg/SWG_Report/SWG_Report_7.2.03.pdf 46 Stellar Populations and Chemical Evolution | GMT Science Case

Group (e.g. the S0 galaxy the Sombrero and the E galaxy Centaurus A), the results will provide broader constraints on galaxy formation than are currently available.

Figure 4.5 Simulated images of a globular cluster at the distance of Centaurus A (3.8Mpc). The cluster has a core radius of 3pc and the field shown corresponds to 2” on a side. The three panels show the cluster as imaged with PSFs appropriate to HST, Gemini and GMT at 1.5m. The GMT simulation uses a PSF with a Strehl ratio of 0.7.

Further, the potential for GMT to obtain integrated light spectra of large samples of globular clusters in individual galaxies will enable a powerful probe of the dynamics and chemical composition of galaxy halos in a wide range of environments. With 8 - 10m telescopes one can probe the dynamics of extragalactic globular clusters with I-band magnitudes of ~22 - 22.5, and can carry out spectral analyses to derive ages for objects 1 - 1.5 mag brighter. With a multi-object wide-field intermediate resolution spectrograph on the GMT, one could probe ~1.5 magnitudes fainter, bringing globular cluster systems in galaxies as far as the Coma Cluster within reach of detailed spectroscopic studies. In this respect, the provision of wide wavelength coverage (e.g. 4000 - 9000Å) in GMT’s multi-object spectrograph brings a number of scientific advantages.

4.6 Star Formation in the Milky Way's 5kpc Ring

The structure and evolution of galaxies are driven by the conversion of gas into stars. Winds, outflows, UV radiation, chemical enrichment, and turbulence of the star formation process affect the physical and chemical structure of galaxies. Dust and gas disks (the remnants of star formation processes) provide the building blocks of planetary systems around stars. The GMT's ability to obtain high-resolution near-infrared spectra and images can thus impact a wide range of astrophysical questions by studying how stars form in the Milky Way.

GMT will be able to probe embedded clusters in ways not currently possible. For example, low-mass stars make up the largest fraction of young clusters, but it is presently very difficult to obtain spectra of these intrinsically faint stars. Near-IR spectroscopy is the best tool for measuring the age, mass, and temperature of young stars, and thus for determining a cluster’s age and initial mass function (e.g., Winston et al. 2009, Gorlova et al. 2010). In addition, emission-line objects can be detected, and brown dwarf members can be identified. Spectra can also probe various diagnostics of the GMT Science Case | Stellar Populations and Chemical Evolution 47 circumstellar disks and outflows. An infrared echelle, such as GMTNIRS, is an ideal instrument for this type of investigation. The 5kpc Ring (a concentration of giant molecular clouds in the disk located 5kpc from the Galactic center and approximately 5kpc from the Sun) is where most stars in the Galaxy currently form, yet it is not well-studied due to its distance and high optical extinction. This molecular gas feature contains ~70% of the molecular gas inside the solar circle (Combes 1991) and most of the Galaxy's far-IR luminosity and giant HII regions (Burton 1976; Robinson et al. 1984). Feedback (in the form of stellar winds and outflows, radiation pressure, and supernovae) from the massive stars in the 5kpc Ring may govern star formation in these regions. These mechanisms are quite different in lower-mass star forming regions studied near the Sun, such as the Orion complex, which is 10 times less massive than a typical giant molecular cloud in the 5kpc Ring. For the first time, GMT will allow us to study star formation in the 5kpc Ring with comparable sensitivity to the mass limits currently achieved in nearby low-mass star forming regions.

One potential GMT target for this type of work is the W51 star formation complex (Figure 4.6). Near-IR observations have found many embedded clusters of young stars in W51, and IR spectroscopy has identified several massive stars in the process of forming, including one of the few O3 stars known in the Galaxy (Barbosa et al. 2008). Massive stars are commonly found in complex environments surrounded by clusters of lower- mass stars and compact HII regions. Angular resolutions of ~0.1arcsec are required to isolate individual objects from nebular emission in order to accurately determine their properties and to model the accretion disks around them. Such a requirement will be met with a LTAO near-IR imager on the GMT.

Figure 4.6 Spitzer three color image of the W51 star formation region from Clark et al (2009).

Additional requirements for effective study of star forming complexes include large fields of view, multiplex capability, and full near-IR wavelength coverage. Giant molecular clouds subtend ~0.5deg and contain thousands of interesting targets. Temperature- and gravity-sensitive features from atomic (Ti, Fe, Si, Mg, Al, Ca) and

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molecular (H2O, FeH, CH4, CO) species are found in the J, H, and K passbands (Cushing et al 2005). Paschen and Brackett lines of Hydrogen and the CO overtone bands can be used as diagnostics of accretion and the conditions in the disks around young stars. At longer wavelengths, thermal emission from circumstellar dust dominates over the photospheric emission from a star, thus GMT observations are complementary to ALMA.

4.7 Summary

GMT will take spectroscopic and photometric studies of stars in the Milky Way and beyond to a new level. This will be facilitated by a suite of instruments allowing for detailed characterization of the Milky Way’s stellar content as well as those of other galaxies. The combination of both will provide exemplary observations to study galaxy formation and evolution in an unprecedented way.

References

Aoki et al. 2006, ApJ, 639, 897 Barbosa et al. 2008, ApJ, 678, L55 Beers & Christlieb 2005, ARA&A, 43, 531 Belokurov et al. 2006, ApJ, 642, L137 Burton 1976, ARA&A, 14, 275 Cayrel et al. 2001, Nature, 409, 691 Clark et al. 2009, A&A, 504, 429 Combes 1991, ARA&A, 29, 195 Cushing, Rayner, & Vacca 2005, ApJ, 623, 1115 Eggen, Lynden-Bell & Sandage 1962, ApJ, 136, 748 Frebel 2010, Astronomische Nachrichten, 331, 474 Frebel & Norris 2012, “Planets, Stars & Stellar Systems”, by Springer, in press Frebel et al. 2007, ApJ, 660, L117 Frebel et al. 2005, Nature, 434, 871 Gorlova, Steinhauer, & Lada 2010, ApJ, 716, 634 Keller et al. 2007, PASA, 24, 1 Kirby et al. 2008, ApJ, 685, L43 Larson et al. 2011, ApJS, 192, 16 McWilliam 1995, AJ, 109, 2757 Norris et al. 2008, ApJ, 689, L113 Norris et al. 2010, ApJ, 723, 1632 Odenkirchen et al. 2009, AJ, 137, 3378 Robinson et al. 1984, ApJ, 283, L31 Salvatori, Schneider & Ferrera 2007, MNRAS, 381, 647 Schatz et al. 2002, ApJ 579, 626 Searle & Zinn 1978, ApJ, 225, 357 Sneden et al. 2003, ApJ, 591, 936 Winston et al. 2009, AJ, 137, 4777

GMT Science Case | Stellar Populations and Chemical Evolution 49

5 Galaxy Assembly and Evolution Casey Papovich, Josh Simon, Karl Gebhardt, Rob Sharp and Patrick McCarthy

5.1 Introduction

Understanding the formation and evolution of galaxies remains one of the key challenges in astrophysics. Sophisticated theories and numerical modeling now provide an environment in which highly refined models can be directly compared to observations. This has led to a deeper understanding of the physical processes that drive galaxy assembly and evolution. Hierarchical cold-flow and hot-flow accretion, mergers involving dissipation (gas-rich or “wet”) or dissipationless (gas-poor or “dry”) mergers, feedback and outflows, and rapid monolithic aggregation are paradigms that are under tension in contemporary theoretical frameworks for understanding the early formation, structural properties, and evolution of luminous galaxies.

Observations spanning the full history of the Universe are critical to developing our understanding of these processes. Locally, dwarf galaxies provide a fossil bed of relics from the low-mass end of the galaxy formation spectrum, and tests for dark matter (Chapter 6), while redshifts between 1 and 4 span the peak of the massive galaxy building era. The early building blocks of galaxies can be studied in situ at z > 6 and this rich subject is considered in detail in Chapter 7.

The GMT will make unique contributions to our understanding of the formation and evolution of galaxies through its ability to obtain spectra of extremely faint objects and to image galaxies with spatial resolution on the order of 100pc at large cosmological distances. Large surveys of the present-day galaxy population provide a firm benchmark for evolutionary studies and sample the population of low-mass dwarfs, while deep survey fields from NASA’s Great Observatories provide sizable samples of galaxies at large and intermediate redshifts. A number of future space- and ground-based facilities will have strong synergy with GMT in this science area, either by providing survey databases or by accessing regions of the resolution-wavelength space not probed by GMT.

5.2 Near-Field Studies of Galaxy Assembly

Dwarf galaxies, and ultra-low-mass dwarfs in particular, provide unique laboratories for studying the building blocks of today’s galaxies. The lowest mass systems present today are believed to be relics of early dark matter halos that have escaped infall into larger systems. These heavily dark-matter dominated systems provide a unique opportunity to study the small-scale properties of dark matter halos and to test the CDM paradigm (Simon & Geha 2007; Geha et al. 2008). Their mass-to-light ratios, space density and the chemical abundances in their few stars provide a window on galaxy formation and evolution.

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Figure 5.1 (Left) The radius out to which detection is complete for faint dwarf galaxies in various surveys (Tollerud et al. 2008). Currently known dwarfs (red circles) delineate the completeness volume covered by the SDSS. (Right) The fraction of the Milky Way halo volume (out to 400kpc) 3 within which a given survey can detect 10 LSun dwarfs as a function of completeness radius. This figure is taken from the Bullock et al. astro-2010 white paper.

The number of known ultra-low mass systems, while still small, has grown rapidly with the advent of large sky surveys (e.g. Willman et al. 2005; McConnachie et al. 2008). Future surveys, and LSST in particular, will greatly increase the available volume for dwarf-galaxy studies. In the near-term, SkyMapper and the Dark Energy Survey (DES) will open the southern sky to dwarf galaxies to depths greater than those of SDSS in the north and the equatorial survey strip. Figure 5.1, reproduced from Bullock et al. (2010), illustrates the survey depths and volumes for current and planned surveys.

The shallow potential wells in ultra-low mass dwarfs make them unique environments for studying the enrichment process. Single supernovae events can leave a clear imprint in the abundance patterns of subsequent stars. The number of stars accessible to detailed abundance work is presently frustratingly small, as Figure 5.2 illustrates. The visible and near-IR echelle spectrographs being developed for the GMT are particularly well suited to this science, and the facility multi-fiber system (MANIFEST) could greatly increase the power of the visible echelle in studying the dark matter content and stellar populations in low-mass systems.

In Figure 5.2 we show color-magnitude diagrams for dwarf galaxies ranging from fairly rich systems (such as Fornax) to those with only a handful of constituent stars (such as Segue 1). The reach of current echelle spectrographs is marked by a fiducial line at V = 19. GMT and the G-CLEF echelle will allow one to reach ~1.5 magnitudes fainter, greatly increasing the number of accessible stars, particularly for the lowest mass dwarfs. Some systems that are beyond the reach of 8m echelle spectrographs (e.g. CVn I, Leo IV) will be accessible for the first time with GMT.

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Figure 5.2 Color-Magnitude diagrams for known dwarf and ultralow mass dwarf galaxies. The current limit for a few hours of integration on an 8m-class telescope is shown by the solid red line. The dashed blue line is the analogous limit for the GMT using the G-CLEF fiber fed echelle spectrograph. The number of stars within reach of velocity and abundance determinations is greatly improved and some systems that are beyond the reach of 8m Echelle spectrographs (e.g. CVn I, Leo IV) will be accessible for the first time with GMT (J. Simon, private communication).

5.3 The Galaxy Building Epoch

The process of galaxy building spanned several Gyr, with the peak period of mass growth occurring between redshifts of ~5 to as low as z ~ 0.5, with the bulk of the mass growth in M* galaxies occurring in the interval from 1 < z < 3. This is considered to be the period of galaxy building—earlier times are considered part of the epoch of first light.

A broad range of processes contribute to the growth of galaxies: conversion of in-situ gas into stars, accretion of outside gas through cold flows, and the build-up of stars and gas through major and minor galaxy mergers. These processes are modulated by feedback from massive stars and supernovae, large-scale galactic winds, and outflows powered by nuclear AGN. Understanding the interplay between these various processes and their impact on the present-day properties of galaxies is one of the most active areas of research today.

5.3.1 Star Formation, Mass Assembly and Chemical Evolution

In the past several years a number of groups have compiled extensive surveys of the stellar mass density in galaxies (Figure 5.3). These compliment surveys of the evolving star formation rate density, the “Madau-Lilly” (Lilly et al, 1996; Madau et al. 1996) plot.

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Two measures of galaxy formation (one, the integral of the rate of conversion of gas into stars, the other a direct measure of the build-up of star formation products and mergers) track each other reasonably well. Systematic shortfalls in the stellar mass density compared to the integral of the observed star formation rate remain and likely result from incompleteness at low masses and uncertainties in the faint-end slope of the luminosity function.

Figure 5.3 Evolution of the stellar mass density history from z ~ 4 to the present compiled from a variety of surveys, as reported in Marchesini et al. (2009). The steady build up of stellar mass in galaxies is clear, but discrepancies between the integral of the star formation rate and the stellar mass history remain.

5.3.2 Dynamical Masses

Theoretical treatments of the growth of structure on galactic scales and greater deal primarily with total rather than baryonic masses. The underlying dark matter distribution drives the growth of galaxies, groups and clusters through accretion of gas along dark matter filaments and through hierarchical merging of structures in over-dense regions. Comparison between theory and observation is greatly facilitated by accurate determinations of the dynamical masses for galaxies, groups and clusters. Several methods are being employed to acquire dynamical masses for galaxies at z > 1. Determining dynamical masses from stellar velocity dispersion measurements is best achieved in the near-IR (where spectroscopy is sensitive to stellar absorption features in the rest-frame optical at redshifts 1.5 < z < 3.5) and in conjunction with high-angular

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resolution imaging to determine sizes and shapes. Near-IR spectroscopic observations are, however, extremely challenging with current instruments on 8 - 10m telescopes and only the most massive, luminous systems are within reach of present facilities.

Figure 5.4 Comparisons between state-of-the-art spectra of red galaxies with simulated spectra from NIRMOS on GMT. The left panels show Gemini/GNIRS (van Dokkum et al. 2009) spectra and VLT/XShooter spectra (van de Sande et al. 2011) of red galaxies with J = 22.5 and 19.9, respectively.

The expected velocity dispersions in passively evolving galaxies selected in near-IR multi-color surveys are expected to be in the 100 - 500km/sec range. Very long exposures with IR spectrometers on 8m telescopes have led to reports of ~500km/sec dispersions in luminous red galaxies at z ~ 2 (van Dokkum et al. 2009), as shown in Figure 5.4.

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The GMT’s large collecting area coupled with an R ~ 3000 - 5000 near-IR spectrograph will allow velocity dispersion measurements with high confidence in massive galaxies, and will enable one to observe L* and sub-L* systems at z ~ 2. In Figure 5.4 we show a comparison between the 29-hour Gemini/GNIRS spectrum and a simulated 3-hour observations with GMT/NIRMOS.

5.3.3 Kinematics of Star Forming Galaxies

Deep HST observations of luminous star-forming galaxies at the peak of the galaxy building era have revealed them to be comprised largely of “clump-cluster” sources and their edge-on counterparts, “chain galaxies” (Elmegreen et al. 2008, Figure 5.5). The kinematics of these galaxies is presently being explored through two-dimensional integral field spectroscopy. These observations, using Hα or [OIII]5007 allow one to map both line-centers and line-widths over the full face of distant galaxies.

Figure 5.5 Examples of rest frame UV/optical images of four galaxies from the HST UDF, ACS V (left of each pair) & NICMOS H (right of each pair), from Elmegreen et al. (2008) showing the clumpy structure of high-redshift galaxies with (upper) and without (lower) clearly defined bulge structures. Observations at the limit of modern instrumentation show tantalizing hints of fundamental structural building blocks of galaxies at this key epoch, but only ELTs will provide the critical combination of sensitivity and resolution necessary to probe these unit building blocks.

While these galaxies often present projected diameters of ~2” (ten times the nominal 200mas slit widths common to instruments such as JWST/NIRSpec or GMT/NIRMOS), they contain substructure on scales of 20 - 30mas or ~1/10 of this resolution. Adaptive optics allows one to sample many independent spatial regions across galaxies that are typically ~1 - 2” in size. The GMTIFS AO-fed IFU spectrograph designed for GMT will have spaxel scales well matched to the sizes of the high surface brightness clumps.

To date, IFU observing programs on 8m apertures have produced some surprising results. The line widths in most of the intense star forming systems are high and there is a broad range of velocity fields. Some systems show apparently well-organized rotation curves

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despite irregular morphologies, while others have disorganized velocity fields or multiple components indicative of ongoing mergers. Most of these observations, however, are badly photon starved. Three instruments are being applied to this effort at present: OSIRIS on Keck (Law et al. 2009), SINFONI on the VLT (Forster-Schreiber et al. 2009) and NIFS on Gemini North (Green et al. 2011). Most groups observing 1 < z < 3 galaxies are using the coarsest spatial scales to improve the photon statistics and a mix of AO and non-AO observations are being used to sample small spatial scales and to produce large samples via high sky coverage.

Figure 5.6 Simulated velocity channel maps from an observation of a z = 1.5 galaxy with the GMTIFS instrument on the GMT. An ordered velocity field with an amplitude of +/- 200km/sec was imposed on an ACS i-band image from the Hubble Ultra Deep Field. This simulation uses data obtained by Bournaud et al. (2008) and assumes 50mas spatial elements and a total line flux of 1.1x10-16 erg cm-2 s-1 (Bournaud et al. 2008) observed over a period of 12h (9h on source +3h on sky).

GMT and other ELTs will improve the photon rates at fixed spatial sampling by an order of magnitude thus allowing robust determinations of galaxy velocity fields over a range of masses and luminosities. GMTIFS will provide a gain in both surface brightness sensitivity and spatial sampling, allowing more detailed studies of high surface brightness systems and access to a broader range of surface brightness levels. In Figure 5.6 we show velocity channels from a simulated GMTIFS observation of a galaxy at z = 1.5. In the model, an organized velocity field has been imposed on a z = 1.51 galaxy from the Hubble Ultra Deep Field. The simulated 50km/sec velocity channels with 50mas spaxels clearly reveal the ordered rotation and the spatial clumpiness in the galaxy.

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5.3.4 The Mass-Metallicity Relation

The evolution of the mass-metallicity and luminosity-metallicity relations provides an important test of galaxy evolution and feedback models. Metallicity in galaxies is determined not just by current and past star formation. Infall of unenriched gas and outflows of enriched gas, particularly during the galaxy assembly era, can strongly impact the evolution of the composition of both gas and stars.

A number of studies over the past several years have explored the evolution of mass- and luminosity-metallicity relations using simple abundance indicators (e.g. the R23 method), and these studies suggest rapid evolution in the mass-metallicity relation out to z ~ 3. Most of these studies, however, are confined to fairly high mass systems at z > 1 or rely on averaging or composite spectra. The middle panel in Figure 5.7 shows the mass- metallicity trend from stacked spectra for nearly 100 z ~ 2 galaxies from Erb et al. (2006). But these data offer no measurement of the intrinsic scatter in the mass metallicity relation, which constrains the strength of feedback and the propagation of metals from galaxies in the intergalactic and circumgalactic mediums (Davé et al. 2011).

Figure 5.7 The evolving mass-metallicity relationship in star forming galaxies in three redshift bins. The blue line shows the present-day relation from SDSS (Tremonti et al. 2004), while the panels show results at z = 0.7 (Savaglio et al. 2005), z = 2.2 (Erb et al. 2006) and z = 3.5 (Maiolino et al. 2008).

NIRMOS on GMT should produce well-constrained gas-phase abundance determinations in large samples of galaxies at select redshifts that put the critical lines ([OII]3727, [OIII]5007, 4959, H, H) in clear atmospheric windows. These samples will provide measures of both the average mass-metallicity and its scatter. JWST and its near-IR spectrograph will make great contributions in this area, as it will be able to survey large samples of galaxies with continuous and broad redshift coverage. GMT will aid in medium-resolution and high-sensitivity observations in regions of the spectrum free of strong atmospheric absorption. The dispersion in the mass-metallicity relation is of particular interest as it offers a test of feedback models. If wind-driven outflows are common during periods of intense star formation then the gas-phase abundances will evolve rapidly and episodically, producing a scatter in the galaxy-to-galaxy properties at a fixed mass.

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5.4 Feedback and the galaxy-IGM connection

Feedback (the injection of energy into gas in galaxies and the resulting regulation of star formation) is now thought to be the key to understanding how present day massive galaxies acquired their distinctive morphologies, masses and stellar content. The intergalactic (IGM) and circumgalactic media (CGM; defined to be the gas within about 300kpc of the galaxies) provide a laboratory in which the feedback effects from galaxy formation and AGN accretion can be measured. One of the promising routes to understanding the relevant baryonic processes is to survey galaxies and gas in the IGM in the same cosmic volumes during the epoch when they were exerting the most influence on one another, near the peak star-forming and black hole accretion era at 2 < z < 4. Studying gas and galaxies in this way provides complementary information on the state of baryons, both those collapsed into galaxies and those residing outside of galaxies.

Recent theoretical work on galaxy growth has focused on the infall of cold gas (“Cold Accretion”), which (in simulations) flows along dense filaments directly onto the central regions of forming galaxies. Theory is reaching some convergence in high-redshift galaxies where this cold gas feeds high SFRs until the galaxy halo achieves some critical 12 threshold (usually around 10 solar masses) and a virial shock develops, suppressing further cold accretion (e.g., Keres et al. 2005, Dekel et al. 2009). The observational signatures of Cold Accretion are thought to be very subtle and perhaps indistinguishable, from outflows. The most telling difference may be found kinematically in the form of absorption lines of infalling (redshifted) gas. The discovery of infalling gas, or any constraint on this process, has important consequences for understanding galaxy evolution.

The intervening absorption-line systems in (background) galaxy and QSO spectra are far more sensitive to low column densities and high ionization states. These provide our best constraints on the extent of cold gas in the CGM around distant galaxies. Furthermore, (rest-frame) UV spectra of distant galaxies allow for detailed study of galaxy-scale outflows and/or inflows of cold gas via strong interstellar absorption lines and Lyman- alpha emission. Current surveys using large samples of spectroscopic data for UV bright galaxies at 2 < z < 3 provide our best constraints for large scale superwind outflows in these galaxies, indicating that winds are a ubiquitous feature associated with star formation, as illustrated in Figure 5.8, taken from Steidel et al. (2010).

It is now possible to demonstrate the basic approach to probing outflows using background sources and derive a statistical determination of the extent of ionized gas around star forming galaxies. This is shown in Figure 5.8 taken from Steidel et al. 2010. These pilot studies conducted with Keck were hampered by several crucial limitations, however. First, due to Keck’s smaller aperture, current spectroscopic surveys (Steidel et al. 2010) are limited to galaxies brighter than ~L*. This does not allow for tests on the relative strength of outflows or the extent of metals in the CGM as a function of galaxy luminosity or mass. (Galaxy formation simulations use different feedback effects, including momentum vs. radiation driven winds, which predict different and testable relations between outflow velocity and galaxy mass/luminosity (e.g., Davé et al. 2011)).

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Second, Steidel was able to probe effectively only a single redshift, allowing no baseline for evolution in the nature of the CGM of galaxies.

Figure 5.8 Composite spectra of star forming galaxies as a function of impact parameter from high surface brightness background galaxies. One can see clear evidence of CIV absorption at distances as large as ~60kpc and Ly to ~100kpc. From Steidel et al. (2010).

Large spectroscopic surveys using GMT with GMACS, which probes both a longer redshift baseline and larger range in luminosity, will greatly extend this work. Specifically, GMACS would obtain spectroscopic observations of a large sample of galaxies from z ~ 1.5 to z ~ 5 and beyond, spanning fully the cosmic volume when galaxies were most active. For comparison, Steidel et al. (2010) obtained spectra for 2500 galaxies at z ~ 2 - 3, with spectroscopy limited to R ~ 24 - 24.5 mag, which yielded a sample of 500 foreground-background galaxy pairs with impact parameters 3 < b < 125kpc at z = 2.2. Surveys with GMT will require similar sample sizes of foreground- background pairs at each redshift we wish to probe (this can be done simultaneously, using galaxies at 3 < z < 5 to probe the CGM at z = 2, etc.).

LBGs have a sky density well-suited to a survey with a wide-field instrument like GMACS. Table 5.1 lists the number of LBGs per GMACS field in a variety of redshift bins.

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Table 5.1 Galaxy populations suitable for galaxy-IGM surveys

Surface Density No. per Limiting Redshift Reference [arcmin2] GMACS field magnitude 2 5.4 780 R = 25.5 mag Reddy & Steidel 2009 3 2.3 330 R = 25.5 mag Reddy & Steidel 2009 4 1.7 240 i = 25.5 mag Bouwens et al. 2007 5 0.22 30 i = 25.5 mag Bouwens et al. 2007

For galaxies at 2 < z < 4, the GMT will probe approximately 1 - 1.5 mag deeper with spectroscopy compared to existing Keck surveys, and be limited only by the physical number of slits available per mask. This will allow not only studies of common “L*” LBGs, but also sub-L* objects (down to ~0.25 L*). A wide-field spectrograph such as GMACS allows for 100 slits per mask available for multiplexing, which will make it possible to obtain samples of over 1000 galaxies each at redshift 2, 3, and 4. At z = 5, the survey will be limited by the source density. Nevertheless, a sample of several hundred galaxies could be obtained with tens of spectroscopic slit masks.

The current expected performance of GMT with GMACS is that it will achieve S/N = 10 for sources at about 24.5 mag at R ~ 2000 in 1 hour. At this S/N, the ISM absorption features become apparent even in these fainter galaxies. Typically, redshifts for LBGs based on absorption lines require S/N ~2 - 3 per resolution element (Steidel et al. 2003), which is doable with GMT/GMACS in about 1 hour for galaxies with 25.1 mag and R ~ 2000, as illustrated in Figure 5.9. Therefore, assembling a sample of galaxies to probe the gas kinematics at a single redshift (e.g. z ~ 2) requires roughly 25 GMACS masks. This will require about 4 nights (assuming 1 hour per slit mask and an overhead factor of 1.5). Extending this survey to a range of redshifts will require a larger investment of time, but will be doable in ~20 nights on the telescope. This science requires that the optical spectrographs have sensitivity over a wide wavelength range (~0.3 - 1 micron) to detect the rest-frame UV spectral features of the gas associated with the ISM and CGM of galaxies for 2 < z < 5.

In addition to studying cold gas in outflows and inflows at 2 < z < 5, these kinds of wide- field optical spectrograph surveys will provide invaluable information for a variety of studies of galaxy evolution in this redshift range, including the evolution of the UV luminosity function (e.g., Reddy & Steidel 2009), evolution of the mass-SFR relation (Stark et al. 2009), and the evolution of stellar populations and extinction (Bouwens et al. 2007), among others.

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Figure 5.9 Simulation of the expected data quality of a galaxy at z = 5 with I = 25.1 AB mag. The top panel shows a simulated spectrum of the galaxy in 1 hour with Keck/DEIMOS where the S/N is ~0.5/pixel and only Lyα in emission would be detected (if present). The bottom panel shows the equivalent spectrum with 1 hour on GMACS, where the predicted S/N is ~2/pixel (~5/resolution element). In this spectrum, galaxy ISM absorption lines are already visible.

The GMT and other ELTs should allow these types of observations for individual objects thereby enhancing our understanding of the baryon cycle as metals are processed in galaxies and into (and through) the IGM. Even with the GMT’s large collecting area these will be challenging observations. Sufficient resolution in the blue end of the spectrum to optimize sensitivity to interstellar lines, and high multiplexing factors are both important to maximizing the return from a large tomographic survey of the IGM at 2 < z < 3. GMACS should be a powerful instrument in this respect. MANIFEST coupled to G-CLEF can also probe sightlines to multiple faint quasars providing a more sensitive, but sparser, sampling than possible with star forming galaxies.

5.5 The Galaxy-Black Hole Connection

One of the most interesting discoveries in galaxy dynamics in recent years is the close connection between bulge dynamics and central black hole masses. High resolution imaging observations with HST, and spectroscopy from the ground and from orbit revealed that supermassive black holes are common in the centers of luminous galaxies. The connection between bulge velocity dispersion and central black hole mass suggests a closely-linked formation and evolutionary process (Figure 5.10). The suspected role of central black holes and AGN in feedback and quenching of star formation makes a deeper understanding of the black hole-galaxy connection all the more pressing.

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Figure 5.10 The local black hole mass-sigma relation from Gultekin et al. (2009). Objects are color coded by the Hubble type of their host galaxies. The central black hole mass correlates strongly with the bulge velocity dispersion. Exploring the extremes of this correlation is a high priority for ELTs and will allow one to discriminate between some classes of models.

Adaptive optics and integral field spectrographs are opening a much broader range of parameter space and survey volume to ground-based telescopes. The need to resolve the black hole sphere of influence and reach surface brightness levels characteristic of stellar bulges make observations beyond z ~ 0.25 very challenging for 8m telescopes. Figure 5.11 shows the angular size of the black hole sphere of influence as a function of mass.

Even with the large gain in collecting area provided by GMT, exposure times required to obtain accurate stellar velocity dispersions are quite long and depend on redshift and available spectral features. For redshifts below ~0.6 the CO band heads offer the best features. H-band spectra with R = 5000 - 10000 should provide sufficient resolution to suppress the impact of atmospheric OH emission and provide enough velocity resolution for accurate mass modeling. In Figure 5.12 we show an example of CO spectra of NGC6086 from McConnell et al. (2011). The strong band heads provide features for either template matching or cross correlation.

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Figure 5.11 The angular size of the sphere of influence as a function of black hole mass. The diffraction limit of GMT in the J- and H-bands is shown by the dotted line.

We expect that exposure times with the GMT and an IFU at z ~ 0.5 will be around 8 hours for a spatial sampling of 12mas. This will Nyquist-sample the K-band PSF in the diffraction-limited mode. Using the finest scale for GMTIFS (6mas) will likely increase the required exposure time. At z ~ 1 the exposure times should be comparable because one can access the strong CaII triplet features in the H-band. At redshifts of z ~ 2 stellar features become very difficult to discern due to the strong surface brightness dimming, but narrow emission lines, particularly [OIII]5007 and Hα, will be very useful velocity tracers.

Figure 5.12 CO-band head spectra for NGC 6086 from McConnell et al (2011). The strong band heads provide a feature for either template matching or cross correlation.

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A well-balanced GMT survey of the evolution of the MBH-σ relation will contain a mix of galaxies at z ~ 0.25, 0.5, and 1, with a sampling of narrow-lined AGN at z ~ 2. Such a program can be carried out in approximately 40 - 50 nights spread over a few years. In Table 5.2 we summarize the observational approach as a function of redshift.

Table 5.2 Primary features for velocity dispersion methods

Redshift Feature Band PSF(mas) PSF(pc) Exposure Time Range 0.0 – 0.1 2.3m CO band heads K 25 40 ~1 hour 0.1 – 0.6 1.6m CO band heads H 12 85 ~8 hours 0.8 – 2.5 H emission H / K 12/25 130/180 ~1 hour 1.8 – 3.5 [OIII]5007 emission H / K 12/25 135/200 ~1 hour

5.6 Summary

The GMT has the ability to impact a broad cross-section of key problems in the study of galaxy assembly and evolution. The combination of large collecting area and wide-field of view in the seeing-limited mode offers a survey capability that other ELTs will find hard to match. This will enable studies of faint objects with large multiplexing gains as well as programs that rely on contiguous areal coverage. The latter includes studies of spatial clustering, IGM tomography and studies of dwarf galaxies.

GMT’s AO modes also provide a powerful tool kit for examining galaxies, ranging from studies of internal structure to high sensitivity near-IR spectroscopic surveys. GMT should allow clean determinations of velocity dispersions in massive galaxies and kinematic maps in star-forming galaxies. Lastly, by working in the diffraction-limited mode GMT will enable studies of black hole masses, demographics and the MBH-σ connection over a range of redshifts.

References

Bournaud, F., et al., 2008, A&A, 486, 741 Bouwens, R. Illingworth, G. D., Franx, M., Ford, H., 2007, ApJ, 670, 928 Bullock, J. S., Kaplinghat, M., Fruchter, A., Geha, M. Simon, J., Strigari, L., & Willman, B. “Dwarf Galaxies in 2010: Revealing Galaxy Formation’s Threshold and Testing the Nature of Dark Matter” Astro2010 white paper Davé, R., Finlator, K., Oppenheimer, B., 2011, MNRAS, 416, 1354 Dekel, A., Sari, R,, Ceverino, D., 2009, ApJ, 703, 785 Elmegreen, B. G., Bournaud, F., Elmegreen, D. M., 2008, ApJ, 688, 67 Erb, D., Steidel, C., Shapley, A., Pettini, M., Reddy, N., Adeberger, K. 2006, ApJ, 647, 128 Forster Schreiber, N. M., et al. 2009, ApJ, 706, 1364 Geha, M., Willman, . Simon, J. D., 2008, ApJ, 692, 1464

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Green, A., et al. 2011, Nature, 467, 684. Gultekin, K., Cackett, E., Miller, J., Di Matteo, T., Markoff, S., Richstone, D., 2009, ApJ, 706, 404 Law, D. R., Steidel, C. C., Erb, D. et al, 2009, ApJ, 697, 2057 Lilly, S. J., Le Fevre, O., Hammer, F., Crampton, D., 1996, ApJ, 460, L1 Kereš, D., Katz, N., Weinberg, D. H., Davé, R., 2005, MNRAS, 363, 2 Madau, P. et al. 1996, MNRAS, 283,1388 Maiolino R., Nagao, T., Grazian, A., et al. 2008, A&A, 488, 463 Marchesini, D. et al. 2009, ApJ, 275, 1277 McConnachie, A. W., et al. 2008, ApJ, 688, 1009 McConnell, N. J., Ma, C.-P., Graham, J., Gebhardt, K., Lauer, T., Wright, S., Richstone, D., 2011, ApJ, 728, 100 Reddy, N., & Steidel, C. C. 2009, ApJ, 692, 778 Savaglio, S., et al., 2005, ApJ, 635, 260 Simon, J. & Geha, M. 2007, ApJ, 670, 313 Stark, D., Ellis, R., Bunker, A., Bundy, K., Targett, T., Benson, A., Lacy, M., 2009, ApJ, 697, 1493 Steidel, C. C., et al., 2003, ApJ, 592, 728 Steidel, C. C., et al. 2010, ApJ, 717, 289 Tollerud, E. J., Bullock, J. S., Strigari, L. E.; Willman, B. 2008, ApJ, 688, 277 Tremonti, C. A., et al. 2004, ApJ, 613, 898 Willman, B., Dalcantaon J. et al. 2005, ApJ, 626, L85 van Dokkum, P. G., Kriek, M., Franx, M. 2009, Nature, 460, 717 van de Sande, J., Kriek, M., Franx, M., van Dokkum, P., et al. 2011, ApJ, 736, 9

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6 Dark Matter, Dark Energy and Fundamental Physics Richard Kron, Megan Donahue, Casey Papovich, and Mike Gladders

6.1 Introduction

Our understanding of the constituents of the Universe and the evolution of structure has improved dramatically over the past two to three decades. This has come about through a combination of theory and multi-wavelength observations of phenomena as diverse as Cepheid variable stars, supernovae, galaxy clusters, and diffuse background radiations. Giant telescopes will continue to play a leading role in cosmological studies through their access to key diagnostics and sensitivity to objects at great distances.

Since the early 1990’s, the cosmic microwave background (CMB), remnant radiation from the Big Bang, has yielded a wealth of information about the geometry and large- scale structure of the Universe. The Cosmic Background Explorer (COBE) first detected the tiny anisotropies in the surface of last scattering that grew to form the structures we see in the present-day Universe such as voids, filaments, superclusters, and individual galaxies within their dark halos. The COBE result was a turning point, demonstrating that the general Big Bang picture, with an initial spectrum of fluctuations that are amplified under the action of gravity, was a useful framework for understanding large-scale structure formation.

Ten years later, the greater resolution of the Wilkinson Microwave Anisotropy Probe (WMAP) maps (Spergel et al. 2003) revealed the power spectrum of these fluctuations on smaller angular scales. The height and locations of the peaks of the acoustic oscillations in that spectrum provided constraints on the flatness of space and the abundance of baryons and dark matter. WMAP observations have also constrained the optical depth at the epoch of reionization and the total mass of neutrinos, quantities accessible to independent verification.

Completely independently, large-scale galaxy redshift surveys have revealed the imprint of baryon acoustic oscillations (left behind after recombination "froze" those oscillations) imprinted in the power spectrum of the galaxy distribution (Eisenstein et al. 2005). This discovery established a new and robust method for measuring cosmic distances, namely a standard rod that complements the standard candles provided by Type Ia supernovae (the apparent brightness of distant Type Ia supernovae directly probe the expansion history and show accelerating expansion (Perlmutter et al. 1999, Riess et al. 1998)). We now have a detailed empirical measure of the expansion history of the Universe and a cosmological model with minimal free parameters and good predictive power that can be tested and refined.

Finally, the last decade in particular has seen the rise of the so-called "concordance model" that simultaneously explains independent measurements, such as the abundance of the light elements and the age of the Universe. In the concordance model, the Universe is spatially flat, and expansion is dominated by a cosmological constant.

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The result of all this progress is that uncertainties in many basic cosmological parameters (e.g. Ho, age, Ωm) have shrunk from approximately one hundred percent to just a few percent (Freedman et al. 2001; Riess et al. 2011; Spergel et al. 2007) in just 20 years.

Despite this progress, however, very basic questions remain unanswered, including the nature of dark matter and dark energy. Where dark matter is concerned, a number of candidate elementary particles are being considered. Strong limits on, or actual detections of such particles are expected to emerge from laboratory experiments such as the Large Hadron Collider or underground detectors in the next decade. With regard to dark energy, some part or all of the observed acceleration of the expansion could be the result of gravity operating over large distances in a way that requires a modification to General Relativity. Ruling out the need for such modification requires observations that sample the dynamics of the Universe (i.e. the growth of structure over cosmic time) and geometric tests are insensitive to this. We also do not know how the relation between pressure and density for dark energy—its equation of state—evolves. A constant equation of state would be consistent with a vacuum energy, described by Einstein's cosmological constant. It is expected that there will be major progress on all of these fronts in the coming decade.

Understanding the formation of structure requires accounting for the visible properties of galaxies and the distribution of baryons in intergalactic space. On one level, by determining how galaxies cluster at different redshifts we can empirically reconstruct the growth of structure, the history of the rate of expansion, and when the baryons formed stars. But for a deeper understanding, we need a proper accounting of the energy and material cycling between gas, stars, and active galactic nuclei (AGN). For this reason, large numerical simulations of the entire Universe that allow an ever-increasing dynamic range in scales have become one of the most important tools in cosmology. For example, to follow dark matter approximately, numerical models only need to compute changing gravitational forces. But to model the Universe completely, models also need to track the behavior of “normal” matter (baryonic particles), which interacts electromagnetically as well as gravitationally. Furthermore, modeling the rich phenomena of shocks, radiative cooling, and the formation of stars and supermassive black holes is critical for producing testable hypothesis based on the current cosmological paradigm.

Extremely large telescopes such as GMT will advance cosmology and address many of these outstanding questions in several ways. First and most obviously, a large collecting aperture is required to detect and characterize galaxies and AGN at the most extreme reaches of the observable Universe. Furthermore, at any given redshift, a larger collecting area allows us to study less luminous galaxies, and therefore study the more numerous and more representative populations at that epoch. A larger collecting area also allows us to assemble spectra with higher resolution, which then provides greater detail about the contents and kinematics of stars and gas in these systems.

Second, the gain in the speed of detecting background-dominated, unresolved sources is 4 proportional to D for diffraction-limited imaging, where D is the diameter of the telescope (see Chapter 1). Structures at high redshift are compact and thus GMT will provide enormous gains in sensitivity compared to current capabilities.

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Synergy with LSST

The combined work of LSST and GMT will greatly advance our understanding of cosmology and the growth of structure on many scales: LSST will map large fields of view and detect large number galaxies and transient sources, and GMT will obtain spectra in the visible and near-infrared, even for exceptionally faint objects. In short, the LSST will discover objects that can subsequently only be identified and characterized with spectra from the GMT and other ELTs.

 LSST and Dark Energy

A specific example is the advancement of the "Stage IV" measurements to constrain the nature of dark energy, one of LSST’s primary goals. Achieving this requires precise measurements of the history of the expansion of the Universe and the growth rate of clustering. The former depends primarily on two powerful geometrical tests: the trend of apparent flux from Type Ia supernovae, and the location (in angle) of the baryon acoustic peak feature in the galaxy power spectrum; both as a function of redshift. Testing the underlying assumptions of gravitational theory, however, is beyond the reach of these tests. For the latter, the two best-known tests of growth of structure use the dependencies of the weak-lensing correlation function on the source redshift, and the frequency of massive clusters on redshift and mass. For all of these methods, the redshifts of the objects of interest (lensing and lensed galaxies, clusters) can be estimated, with varying confidence, from measurement of colors and photometric redshifts.

Figure 6.1 Redshift distribution for 3621 galaxies with IAB < 24 from the VLT Very Deep Survey (VVDS) as presented in Le Fevre et al. (2005).

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Photometric redshifts can be quite precise, approaching a few percent rms error; generally sufficient for the statistical tests mentioned above. However, it is vitally important that there be no appreciable systematic error, since a biased n(z) will translate directly to an error in the expansion rate. These types of systematic errors, as well as catastrophic photometric redshift errors, are a concern because the detection limit of LSST imaging will probe redshifts and luminosities beyond those that have been well calibrated with spectroscopy. Hence, there is a continuing need for spectroscopic redshift surveys (e.g. Figure 6.1) that are complete to the LSST detection limit and that are large enough to sample the full range of galaxy types with good statistics. GMT can provide these data with the optical and near- IR multiple-object spectrographs. GMACS has a field-of-view of 9 x 18 arcmin and is expected to reach S/N = 5 per resolution element in a one-hour exposure at 0.4μJy. Similarly, NIRMOS has a field-of-view of 6.5 x 6.5 arcmin and reaches S/N = 5 in the H-band in a one-hour exposure at 0.7μJy. These fluxes are brighter than the LSST limit (see Figure 9.6), but the presence of emission lines will enable redshifts to be obtained even at much fainter levels. An investment of 10 nights will yield on order 20,000 redshifts, where the two spectrometers would sample different targets at different redshifts.

 LSST and Clusters of Galaxies

The fields of view of the two multiple-object spectrometers proposed for GMT (GMACS and NIRMOS) are larger than the virialized cores of rich clusters of galaxies, enabling the cores of clusters and their infall regions to be targeted together. LSST will be able to discover the highest density (and therefore the rarest) regions in the cosmic web for galaxies over a range of redshifts. GMT will then efficiently map the infall velocity fields in these regions, fields that can then be compared directly with N-body simulations. Furthermore, identifying and fitting the caustics in the distribution of galaxy velocities around a cluster provides a mass estimate out to the virial radius and beyond (e.g. Andreon 2010). This in turn yields a cluster mass measurement and profile, independent from mass profiles determined from gravitational lensing and from hot gas (X-rays and Sunyaev-Zel'dovich Effect).

 LSST and Transient Sources

LSST is expected to detect and characterize hundreds of thousands of extragalactic transient sources each year, mostly supernovae. This rich mine of information will be analyzed as functions of redshift, type, host galaxy properties, and environment of the host. Type Ia's SNe, in particular, are considered to be ‘standardizable’ candles, but it will be important to continue to investigate any intrinsic evolution in the properties of the objects selected as candidate Type Ia's. GMT's efficient spectrographs will provide R ~ 3000 spectra up to z ~ 1.5 - 2 and enable a detailed assessment of how such supernovae may differ from those at lower redshift.

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6.2 Clusters and Dark Matter

The distribution of mass within rich clusters of galaxies is diagnostic of the history and dynamical details of the assembly of these large structures. The value of the cosmological parameter σ8, the normalization of the matter power spectrum to 8Mpc scales, is exquisitely sensitive to the number of high-mass clusters of galaxies and the mean density of matter, Ωm, while the rate of evolution of the cluster mass function depends powerfully on Ωm. Therefore, cluster numbers and their evolution have proved to be reliable tools for probing the world-model and key cosmological parameters. The internal structure of clusters reflects dynamical processes that link baryons and dark matter (and potentially dark matter self-interaction). Studies of great bubbles in the hot intra-cluster medium around central cluster galaxies have offered the first estimates of the total energy output of AGN. Studies of clusters of galaxies and their contents, therefore, are relevant to a broad range of questions in extragalactic astronomy.

Figure 6.2 Mass map for Abell 1689 superposed on deep g, r, i, images from HST/ACS. Spectroscopy of faint lenses allows more accurate mass models. From Coe et al. 2010.

The technique of measuring the shapes and flux magnifications of distorted images of background galaxies, measured statistically over large numbers of galaxies behind clusters is known as weak lensing. A map of the weak lensing shear field around a cluster of galaxies reveals the overall mass surface density in clusters (e.g. Figure 6.2). HST is providing such data for clusters at intermediate redshift, but only a few dozen clusters are likely to be mapped by the end of the HST mission, not enough to cover the anticipated range of properties as a function of both total mass and redshift. With ground-layer

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Adaptive Optics (GLAO), GMT can image fields up to ~9 arcmin across with PSF FWHM of the order of 0.25 seconds of arc. The J- and H-bands have the best signal-to- noise ratio for 1 < z < 2. For example, at z = 1 a field of 8 minutes of arc corresponds to co-moving 3.8Mpc enabling the weak-lensing signal to be followed far enough out to connect to the background surface density.

The best constraints on dark matter distribution in clusters via weak lensing come from the detailed redshift distribution of the lensed sources. Information from photometric redshifts can be enhanced with direct spectroscopic calibration in the same fields, but having substantial numbers of the weak-lensed source galaxies with spectroscopic redshifts is even better. Galaxies in the background of a cluster at z = 1 are faint; effective studies of the lensed populations requires both the aperture and the field-of-view of GMT.

Figure 6.3 Example of a strong cluster lens from the SDSS survey (SDSSJ1038+4849). Four galaxies with redshifts ranging from 0.8 to 2.8 are each imaged multiple times by the rich foreground cluster. The location and shapes of the various lenses images can be used to map the distribution of dark matter in the cluster and compare it with the distribution of luminous matter. See Bayliss et al. (2011) for details.

Strong lensing (that which produces the giant arcs seen in a subset of rich clusters such as SDSSJ1038+4849 in Figure 6.3), provides complementary and important information about mass distribution that probes down to the scale of the cores of clusters. Thus the combination of weak and strong lensing allows one to measure substructure and the concentration of the mass in the clusters, as well as the overall ellipticity of dark matter in the clusters. In at least some cases, the central concentration appears to be higher than

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predicted by ordinary CDM, possibly suggesting an earlier formation for these structures (Coe et al 2010), or perhaps indicating the dynamical effect of baryon dissipation (Oguri et al. 2011).

A number of observations/methods are required to study the evolution of the assembly of clusters, e.g.:

 Probing a broad range of cluster masses at higher redshifts  Sampling many clusters of similar mass to treat projection effects, and  Obtaining redshifts for as many arcs as possible to constrain the mass distribution

Abell 1689 (Figure 6.2) is an example of a cluster in which numerous arcs have been identified (135 arcs from 42 distinct sources; Coe et al. 2010). Most of these arcs are beyond the reach of spectroscopy with 8m telescopes, but can be studied with the GMT. Samples of strong lenses will come from LSST for z < 1 and from Euclid for z > 1. eROSITA will provide catalogs of X-ray bright clusters that can be followed up in the near-infrared, which may be another effective way to find giant arcs.

More speculatively, dark matter self-interaction may be revealed in the cores of dark matter haloes (Spergel and Steinhardt 1999), including the centers of rich clusters of galaxies. Miralda-Escude (2002) pointed out that the ellipticity of dark matter in MS 2137-23 is elliptical even at a radius of 70kpc, which constrains this possibility. Even so, strong lensing due to small-scale concentrations of dark matter can be used to test dark matter interaction models. Multiple lines of sight to a distant AGN, for example, can be used to probe small-scale structure in dark matter haloes in massive foreground galaxies (Keeton and Moustakas 2009). Here, the idea is to measure the relative time delays, fluxes, and positions of the images of the source as a function of time, which can then be used to measure the gravitational potential and its gradients that then constrain the substructure mass function. Most of the suitable multiply-imaged AGN's have image separations of an arc second or less. An IFU with GMT would provide the requisite precision measurements.

Searching for the rare instances of highly magnified images of galaxies at large redshifts provides an important opportunity to study the sources (as opposed to the lenses), i.e. using the magnification to examine detailed properties of galaxies at z = 3 and beyond. Magnifications of 3 to 4 magnitudes for sources near z ~ 2 are sufficient to make observations of line strengths and widths (Wyuts et al. 2011). Such strong lenses thus provide the boost in signal-to-noise ratio needed to characterize the stellar populations (e.g. age and star-formation rate) and the internal extinction. The magnification can also be exploited in the spatial domain: a few known cases (e.g. Gladders et al. 2011; Zitrin et al. 2011) provide a basis for forecasting what will be possible with GMT and an IFU working at the diffraction limit. Spectra can be obtained for individual giant HII regions (spatial scale ~200pc) potentially out to z = 5 that are good enough to measure ionization and abundances. These observations are clearly photon-starved, but with larger spaxels it may be possible to measure the underlying stellar continuum.

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Figure 6.4 Dark matter density structure of the progenitor of a z = 0 massive galaxy cluster. Panels A and B show the filamentary structures for cold and warm dark matter while panels C and D show the temperature and density distributions for the warm dark matter filament highlighted by the box in panel B at a redshift of 23. From Gao and Thuens (2007).

6.3 Dark Matter Profiles in Dwarf Galaxies

Measuring the mass function of dwarf galaxies provides fundamental constraints on the properties of dark matter. The predicted number of ultra-faint dwarf galaxies expected to be discovered in future surveys such as the Dark Energy Survey (DES) and LSST vary by a factor of three based on different assumptions for the properties of dark matter and galaxy formation, and range from 3 to 9 per 1000 square degree surveyed to a limiting magnitude of r ~24 - 24.5 (Tollerud et al. 2008). Since the current sample of well-studied ultra-faint dwarfs is fewer than a dozen, characterizing the mass function of newly discovered dwarf galaxies will greatly improve constraints on the properties of dark matter.

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Another test of the properties of dark matter comes from the shape of the dark matter density distribution. Cold Dark Matter (CDM) predicts that dark matter halos should show steep central density “cusps” (Navarro, Frenk & White 1996, NFW). Dwarf galaxies are ideal test subjects to measure dark matter profiles as they are highly dark- matter dominated (with mass-to-light ratios approaching 1000). Since the baryons make a negligible contribution to the mass even in the inner regions, the dark matter dictates all kinematics.

An ongoing controversy driven by data is whether dark matter halos exhibit “cusps” or show shallow density-profile “cores”. Generally, the dark matter halo density can be parameterized by a generalized Hernquist (1990) profile, where the classical NFW profile is a specific case. Walker et al. (2009) compared the dark matter density and kinematic properties of known dwarf galaxies. As illustrated in Figure 6.5, the current data allow for mass profiles including a cuspy NFW profile as well as a cored halo. The current limits are restricted by both the number of dwarf galaxies studied and the number of stars per galaxy for which we have accurate kinematic information.

Figure 6.5 The left panel shows the mass interior to the half-light radius and the right panel shows the mean density within the half-light radius for dwarf galaxies (adapted from Walker et al. 2009). The curves show the best-fitting mass profiles, including the NFW profile with a cusp ( = 1) and a cored model ( = 0). Currently both dark matter mass profiles are consistent with the observations.

A wide-field optical spectrograph such as GMACS will provide a technological leap to measuring the kinematic properties of dwarf galaxies near the Milky Way. The corrected field of view of the GMT (20 arcmin) compares favorably to the size of dwarf galaxies, which are expected to range in diameter from 3 to 30 arcmin. Confirming that dwarf galaxies are gravitationally self-bound systems generally requires kinematic information of at least 100 stars per object down to R = 23 mag at S/N = 5. Since dwarf galaxies are low mass objects, their internal velocities are small and subsequent kinematic

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observations require precision on the order of 3km/s, setting an observational spectral resolution requirement of R ~ 6000. This could be achieved on the GMT with MANIFEST feeding an Echelle, such as G-CLEF, or by an echellette mode for GMACS.

Experience with Keck/DEIMOS has shown that achieving such precision requires 150- minute exposures for R = 21.2 (Simon & Geha 2007), allowing a survey speed of 300 stars with this magnitude per night. Because of GMT’s large collecting area and GMACS’ greater sensitivity, equivalent observations with GMT+GMACS will require only 35 minutes. This capability, plus GMACS’ large field of view translates to a factor of five improvement in survey efficiency, enabling the measurement of up to 1500 stars per night.

GMT will have the ability to measure kinematic information even for ultra-faint dwarf galaxies out to 400kpc, the virial radius of the Milky Way. Given that confirmation of dwarf galaxies requires only a hundred stars per object, one can envision a survey of DES- and LSST-selected dwarf candidates where GMT could observe up to 15 candidates per night. Engaged in such work, GMT will double the number of known dwarfs in a few nights, providing kinematic data to measure the velocity dispersion and thus the dark matter mass profile of dwarf galaxies.

6.4 Summary

GMT’s two wide-field multi-object spectrographs will provide exceptional gains in our ability to investigate dark matter and dark energy via radial velocities of faint stars in dwarf galaxies, and spectroscopy of faint galaxies at high redshift. Weak lensing measurements of background sources lensed by rich clusters of galaxies can be made with GLAO. Selecting highly magnified strong lenses discovered in wide-field surveys will allow detailed, spatially resolved studies of star-forming regions at high redshift.

References

Andreon 2010, MNRAS, 407, 263 Bayliss, M. et al. 2011, ApJ, 193, 8 Coe, D. et al. 2010 ApJ 723, 1678. Eisenstein, D. et al. 2005 ApJ 633, 560 Freedman, W., et al. 2001, ApJ, 553, 47 Gladders, M. et al. 2011 Keeton, C.S. and Moustakas, L.A. 2009 ApJ arXiv: 0805.0309 Le Fevre, O., et al, 2005, A&A, 439, 845 Miralda-Escude, J. 2002 ApJ, 564, 60. Moustakas, L. 2009, arxiv:0902.3219 Oguri, M. et al. 2011 arXiv: 1109.2594v1 Perlmutter, S. et al. 1999 Phys. Rev. Lett. 83, 670 Riess, A. et al. 1998 AJ 116, 1009 Smoot et al. 1992 ApJ 396, L1

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Spergel, D. N. and Steinhardt, P.J. 1999, Phys. Rev. Lett. 84, 3760 Spergel et al. 2003, ApJS 148, 175 Spergel, D. N., et al. 2007, ApJS, 170, 377 Walker, M. et al. 2009, ApJ, 704, 1274 Wyuts, E. et al. arXiv:1110.2833v1 Zitrin, et al. 2011, submitted to ApJ.

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7 First Light and Reionization Steven L. Finkelstein, James S. Bolton, and Casey Papovich

7.1 Introduction

A small number of seminal events mark sharp and distinctive transitions in the history of the Universe: the end of inflation, the quark-Hadron phase transition, recombination, and reionization. The latter, the time when the intergalactic medium (IGM) was reionized, is intimately connected to the formation of the first massive collapsed objects and the birth of galaxies. Exploring the era when galaxies first formed has been among the forefront goals of extragalactic astronomy since the discovery of the hot Big Bang.

Figure 7.1 A graphical history of the Universe (Robertson et al. 2010). Key events relating to the early history of the Universe – recombination, the cosmic dark ages, the birth of galaxies and reionization of the IGM are illustrated. The GMT will allow us to probe the formation of the first galaxies, near the end of the dark ages.

The search for the first galaxies, both from empirical and theoretical perspectives, has been underway since the 1970’s and we are now able to observe and crudely characterize galaxies in the first few hundred million years after the Big Bang. Our theoretical understanding of the conditions in the early Universe, the growth of density perturbations, and feedback from the first generation of stars has also advanced dramatically in recent years. The GMT, working in conjunction with JWST, SKA and other facilities, will provide new and powerful observational tools for studies of the reionization epoch and the period of early galaxy growth that followed.

In this chapter we illustrate a few areas in which the GMT is expected to have a significant impact and highlight ways that GMT instruments and other facilities can work together.

7.2 The First Dark Matter Halos, Stars and Galaxies

State-of-the-art simulations suggest that the first stars formed in mini-halos (gravitationally bound clumps of dark matter) at redshifts between z ~ 30 and 20. However, these mini-halos likely formed few stars, since the ionizing radiation from a

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single star in such a small halo may be able to dissociate and ionize molecular gas and, in some cases, unbind the halos altogether. The enriched material from these first stars seeded subsequent generations of star formation. These detailed numerical simulations show that gas heated by hot stars and the first supernovae eventually cooled and collapsed to initiate a more prolonged episode of star formation, perhaps by z ~ 20 - 15 (see Figure 7.2). Those galaxies which had already hosted one or more primordial stars will have their gas enriched sufficiently to begin forming low-metallicity (Population II) stars. With their cooler surface temperatures, these stars would not have been able to heat and disrupt the gas reservoir as effectively as the previous Population III stars (e.g., Bromm et al. 2009 and references therein).

Figure 7.2 Numerical simulations of the first stars by Tom Abel3 and collaborators. The left panel shows a single primordial 30MSun star at z = 20 and spans a scale of 1 light month (5400 AU). The right panel shows a young massive binary with a separation of only ~800 AU. Massive binaries like this will merge on short time scales and the resulting supernova explosion will seed the surrounding medium.

Empirical characterization of the first galaxies will require observations in the near- infrared (NIR). Early-galaxy candidates will be provided by deep imaging surveys, either from the ground, or more likely from space (e.g. JWST). Deep NIR spectroscopy is needed to determine redshifts for these early galaxies. Only then can we robustly measure the rest-frame ultraviolet (UV) luminosity functions, and infer their contribution to reionization. Near-infrared spectroscopy is also crucial to probing these galaxies for the presence of metal-free stars via high-ionization emission lines such as HeII1640. High- resolution optical and NIR spectroscopy of bright, high-redshift objects such as QSOs or gamma-ray bursts will allow more detailed investigations of the ionization state of the IGM well into the epoch of reionization. The GMT, together with its spectrographs (e.g. NIRMOS, G-CLEF, GMTNIRS, and GMTIFS) will help revolutionize the field of very- high-redshift astrophysics, allowing detailed measurements of gas phase physics to be made in the very early Universe.

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7.3 Galaxies in the Early Universe

Galaxies at large redshifts are generally selected on the basis of their flat (in fν) rest frame ultraviolet continuum emission (Lyman Break Galaxies or LBGs), or their strong Lyα emission lines (LAE). Until recently, the most distant galaxies discovered with these techniques had redshifts limited to z ~ 6 - 6.5, corresponding to ~1Gyr after the Big Bang. This was a practical limitation: at z > 7, the spectral features used to select these galaxies shift into the NIR where the bright night sky makes ground-based discovery of these distant galaxies difficult. The deployment of Wide Field Camera 3 (WFC3) on Hubble in 2009 opened the z > 7 Universe to direct observation, revealing continued decline in global star formation rate at the highest redshifts (see Figure 7.3). The deepest ever NIR images were obtained with WFC3 in the Hubble Ultra Deep Field (HUDF), which led to the discovery of the first large samples of galaxies at z ~ 7 - 8 and, perhaps z ~ 10 (Oesch et al. 2010; Bouwens et al. 2011). The characteristic UV absolute magnitude (M*UV) of these galaxies is fainter than that at z < 6, supporting a strong evolution in the luminosity density at early times (Dickinson et al. 2004). The ultraviolet spectral slopes of the earliest galaxies are quite blue, suggesting that they may host extremely metal-poor stars (Bouwens et al. 2010; Finkelstein et al. 2010), though their colors are consistent with some of the bluest local galaxies.

Figure 7.3 The evolution of the UV luminosity density and derived global star formation rate from z ~ 10 to the present day (from Bouwens et al. 2011). The rate of star formation grew rapidly in the first ~1 billion years after the Big Bang and peaked when the Universe was around 2 - 3 Gyrs old. Observations with the GMT and the James Webb Space Telescope will allow us to probe the first ~500Myr for signatures of the reionization.

As these galaxies are identified on the basis of photometry alone, their redshifts are not secure; a non-negligible contamination rate can lead to biased determinations of luminosity densities and other key parameters. Spectroscopic redshifts are crucial to accurate measurements of the rest-frame UV luminosity function. Current determinations of the UV luminosity function at z ~ 7 - 8 suffer degeneracy between the characteristic magnitude M*UV and the faint-end slope, α. While increased numbers of studied

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examples will help push these uncertainties down, the inferred luminosity functions depend critically on the adopted redshift distribution. Directly measuring this distribution, as well as the contamination rate, will significantly improve the measured luminosity functions. The great sensitivity of GMT’s near-IR spectrographs will allow direct determinations of the faint-end slope from large spectroscopic samples. This will critically impact reionization studies, as a robust estimate of the luminosity function is crucial to determining the specific luminosity density for ionizing radiation from the galaxy population, and thus their contribution to reionization. Current luminosity function constraints leave the specific luminosity density uncertain by a factor of ~10 - 50, at z ~ 7 - 8, which can significantly alter the amount that these galaxies contribute to the ionizing photon budget. This uncertainty is not reflected in the statistical analysis of the evolving UV luminosity density shown in Figure 7.3.

At the time of this writing, only three galaxies at z > 7 have been spectroscopically confirmed via Lyα emission (Vanzella et al. 2010), with only one being at z > 7.1: the z = 8.56 galaxy UDFy-38135539 first confirmed by Lehnert et al. (2010; see also the z ~ 8.2 gamma-ray burst by Tanvir et al. 2009). This galaxy was observed with the SINFONI integral field unit NIR spectrograph on the 8.2m VLT telescope, and required a ~15 hour integration to obtain a ~6-sigma Lyα detection. This significant investment of exposure time was necessary because the galaxy is extraordinarily faint. In the near future, a number of multi-object spectrographs on 8 - 10m class telescopes (e.g., Keck/MOSFIRE, Gemini/FLAMINGOS-2 and VLT/KMOS) will increase the efficiency of these studies with their multiplexing capability. However, they will still be limited by the light- gathering power of their telescopes, requiring ~2 nights of observations to reach the flux level of the brightest Lyα emission lines at z > 7.

Figure 7.4 Left: VLT/SINFONI 15h spectrum of the z = 8.56 galaxy, UDFy-38135539 (from Lehnert et al. 2010). Right: A simulated GMT/NIRMOS spectrum of the same object with the same exposure time in black, with the 1-sigma noise spectrum in red. The significance of the detection in the simulated GMT spectrum is ~70 sigma and the input Ly flux is 6 x 10-18 erg/sec/cm2.

In Figure 7.4 we compare the Lehnert et al. observed spectrum of UDFy-38135539 along with a simulated observation of the same galaxy with the multi-object NIR spectrograph (NIRMOS) on the GMT. Not only is the significance of the detection vastly improved,

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but also the sensitivity is sufficient to allow useful searches for other features (e.g. HeII1640, see Section 7.4). The GMT with NIRMOS will be able to confirm the redshifts and obtain high quality spectra of the brightest galaxies at z > 7 in less than an hour. The star-forming properties of the currently observed z > 7 galaxies imply that (assuming an -18 2 ionized IGM) the majority have Lyα emission line fluxes ~1-2 x 10 erg/sec/cm , or a factor of three-to-six fainter than UDFy-38135539. NIRMOS on the GMT will detect -18 2 4 Lyα at 2 x 10 erg/sec/cm in 10 sec at 10-sigma significance.

HST, even with WFC3, is probably not sensitive enough to peer beyond z = 10 into the era where we might expect the first galaxies to be forming. We expect that the James Webb Space Telescope (JWST) will detect galaxies to z ~ 12, if they exist. The GMT with NIRMOS will have the sensitivity to detect Lyα emission from galaxies at z ~ 12. At this redshift, a galaxy with the same intrinsic Lyα luminosity as UDFy-38135539 will -18 2 have a Lyα flux of 3 x 10 erg/sec/cm , and thus be detectable with the GMT in a reasonable exposure time if the IGM is reasonably transparent at these wavelengths. The GMT with NIRMOS will thus be crucial for confirming galaxies at the highest redshifts, which will place much stronger constraints on estimates of the rest-frame UV luminosity functions, and theories of the formation of galaxies.

Although JWST will have a light-gathering power a factor of ~13 times less than the GMT, it will not be subject to the night sky background that affects ground-based observations. The near-IR spectrometer on JWST (NIRSPEC) will perform R ~ 100 and 1000 spectroscopy over a 9 square arc-minute field-of-view (FOV). This FOV is four times smaller than NIRMOS, thus the GMT will be able to simultaneously observe more targets than JWST to a common flux limit (Table 7.1) Additionally, the moderate resolution of NIRMOS results in narrow sky emission lines, so in the majority of cases emission lines will only be subject to the interline background. In these cases, GMT/NIRMOS will be significantly more sensitive than JWST/NIRSPEC in either its R = 100 or R = 1000 mode. In practice, NIRMOS should be a factor of 2 - 3 times more sensitive than the R = 100 mode in the 1.1 - 1.5μm interval, corresponding to Lyα at 8 < z < 11. Velocity resolution corresponding to R ~ 3000 is required for studies of the spectral morphology of the Lyα emission line and associated self-absorption, which can be used as a diagnostic for the neutral state of the IGM (e.g., Laursen et al. 2010).

Table 7.1 Relative numbers of objects per GMT/NIRMOS and JWST/NIRSPEC field

Redshift Flux Limit n n (GMT/NIRMOS) (JWST/NIRSPEC) 6.0 x 10-18 40 10 3 3.0 x 10-18 130 34 6.0 x 10-18 4 1 7 3.0 x 10-18 20 5

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Using the z ~ 3 Lyα luminosity function (Gronwall et al. 2007) as a baseline we might expect that at z ~ 7 - 10 there will be ~40 galaxies with Lyα line fluxes > 6 x 10-18 erg/sec/cm2 (corresponding to 1.5 - 3.5 L* at z = 7 - 10) in one NIRMOS FOV, while only ~10 such galaxies in a NIRSpec field. At z > 7 the IGM was almost certainly more neutral than it was at z ~ 3 (e.g., Kashikawa et al. 2006; Ouchi et al. 2010). Thus if the characteristic number density and luminosity, Φ* and L*, are reduced by a factor of two at this epoch, only ~4 such galaxies will reside in one NIRMOS field, while only one galaxy will be observable per NIRSpec field. Going a factor of two fainter increases the surface density by a factor of ~5, since we are still on the exponential portion of the luminosity function. Thus, GMT/NIRMOS will be very efficient at studying the bright end of the Lyα luminosity function out to the highest redshifts.

7.4 Discovering the First Stars

Although by z ~ 15 the average metal abundance of the Universe will likely have increased above the critical value necessary for Population III star formation, Pop III stars could still form in primordial gas clouds in under-dense regions, possibly down to currently observable epochs (e.g., Scannapieco et al. 2003). One possible tracer of Pop III star formation is recombination emission from HeII, since photons energetic enough to doubly ionize helium require hotter stellar photospheres than are possible from normal stars. The HeII1640 emission line is one of the most promising Pop III tracers, as it can be very bright, and it is shifted into the optical window for z > 1.5, and into the NIR for z > 5 (e.g., Tumlinsun & Shull 2000). Galaxies hosting Pop III star formation will also be very bright in the Lyα emission line, as more hydrogen ionizing photons are created as well. Thus, dual Lyα-HeII emission is a useful signature of galaxies hosting Pop III stars. A number of studies have performed this analysis at z < 5, but no Pop III hosting galaxies have been found (e.g., Nagao et al. 2008; Wang et al. 2009). Scannapieco et al. (2003) 42 predict that ~30% of L(Lyα) > 10 erg/sec galaxies at z = 6 - 10 host Pop III star formation, thus we likely need to push to higher redshifts.

Table 7.2 Ly fluxes and Ly/HeII flux ratios for different abundances and IMFs

IMF F(Lya) (1E-18) Lya/HeII Z = 0, Top-Heavy 85.0 8 Z = 0, Salpeter 8.5 40 Z = 0.0005, Salpeter 8.5 80,000

The GMT will be a powerful facility for these studies, as NIRMOS will have the ability to detect HeII emission for z > 4.5. In Figure 7.5 we use the most recent models from Pawlik et al. (2010) to estimate the likely levels of HeII1640 emission from Pop III star forming galaxies at high redshift. They consider three stellar populations: zero metallicity with a top-heavy initial mass function (IMF); zero metallicity with a Salpeter IMF; and Z -3.3 7 = 10 with a Salpeter IMF. For a z = 9 galaxy with M* = 10 MSun, these three models correspond to the Lyα fluxes and Lyα/HeII flux ratios shown in Table 7.2.

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Clearly the third scenario (a Salpeter IMF with a non-zero metallicity) will not produce a detectable level of HeII flux, but the two metal-free IMFs may. To assess the ability of GMT/NIRMOS to detect HeII emission from high-redshift Pop III-hosting galaxies, we 7 ran the simulation shown in Figure 7.5. It shows the spectrum of an M* = 10 MSun galaxy at z = 9 with a zero metallicity and a top-heavy IMF obtained in a 2h NIRMOS integration. The simulation indicates that the GMT will be able to detect HeII emission if the IMF is top-heavy. If the IMF is ''normal'', however, HeII will be difficult to measure (unless the galaxy is much more massive), as it is not detected even in an ultra-deep exposure.

Figure 7.5 Left: Model of metal-free disk formation at high redshift (from Pawlik et al. 2010). Right: A simulated NIRMOS 2h integration of a z = 9 metal-free galaxy with a top-heavy IMF. HeII 1640 emission is clearly detectable. With a Salpeter IMF, this same galaxy is not detectable in HeII, thus a top-heavy IMF may be necessary to detect HeII at the highest redshifts with the GMT.

While JWST will be very sensitive to the presence of HeII emission in its R = 100 mode, spectrally resolving this line is desirable for a number of reasons. First, the mere presence of HeII emission does not necessitate the presence of Pop III stars. Wolf-Rayet stars can result in HeII emission as their strong winds expose their hot interiors releasing the necessary high-energy photons for the creation of HeII emission. However, these winds will result in a broadening of this emission line, thus intermediate resolution spectroscopy will be required to properly measure the line profiles. A similar analysis will be required to rule out AGN as causes of HeII emission.

Secondly, emission lines can also be used to estimate the dynamical masses of these galaxies. While Lyα emission will be brighter, the Lyα line profile is distorted by its passage through interstellar and intergalactic media, and therefore is not a reliable tracer of the dynamical mass. In a Pop III star-forming galaxy, HeII will be the next brightest emission line, thus resolved spectroscopy of this line can provide dynamical mass measurements out to the highest redshifts, which will provide key observations to compare to current ΛCDM based models. GMT and JWST will provide the critical spectroscopic capabilities needed to apply these diagnostics to faint galaxies at the highest redshifts.

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7.5 Probing Reionization

7.5.1 Constraints from Ly Emission

Observations of the polarization of the cosmic microwave background imply that the process of reionization started at z = 11 and ended with a largely ionized Universe at z = 7. Determining when and how neutral hydrogen in the IGM was reionized is an important question for observational cosmology and a precursor to understanding the energetics of the first galaxies (Robertson et al. 2010). One of the most practical probes of reionization is the frequency of the occurrence of Lyman-α photons, a clear signature of ionizing radiation. As Lyα photons are resonantly scattered by neutral hydrogen, the abundance of galaxies with Lyα emission should decrease as observations probe into the era where there is neutral gas (e.g., Malhotra & Rhoads 2004). The frequency of Lyα emission in galaxies during reionization is sensitive to IGM neutral fractions of 10 - 100% (McQuinn et al. 2007), which is very complementary to other measurements of the IGM from QSO sightlines and polarization of the CMB.

Recent studies using 8 - 10m telescopes have focused on measuring the space density of UV luminous galaxies, selected on the basis of the spectral break short-ward of Lyα caused by absorption from neutral hydrogen along the line of sight (e.g., Giavalisco et al. 2004).

Figure 7.6 The redshift evolution of the fraction of LBGs with Lyα emission and EW > 25Å, X(Ly), from Stark et al. (2010, 2011) and Schenker et al. (2011, astro-ph:1107.1261). There is tentative evidence of a sharp decline in the occurrence of Ly emission between z = 6 and z = 7, which corresponds to a rapid increase in the neutral hydrogen fraction at z > 6.

Lyman Break Galaxies (LBGs) can be selected over a broad range of redshifts. Stark et al. (2010, 2011) measured evolution of the Lyα EW distribution of LBGs at 4 < z < 7 using ultra-deep (6 - 12h) observations with Keck/DEIMOS and the refurbished LRIS-R. Their result is illustrated in Figure 7.6. From z = 4 to z = 6, the fraction of LBGs with EW (Lyα) > 25Å increases steadily, both with increasing redshift and with increasing luminosity. To z = 6 there is no indication that the IGM is rapidly evolving toward neutrality.

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Recently, several attempts have been to made to extend these measurements to z > 6 galaxies using ultra-deep (12 - 18h) exposures with optical and near-IR spectrographs on Keck and the VLT (Schenker et al. 2011; Fontana et al. 2011, Ono et al. 2011, Vanzella et al. 2011), all at the technological limit of current telescopes and instruments. These studies find few objects with detectable Lyα emission at these redshifts. Figure 7.6 shows that the current sensitivity limits tentatively suggest a drop in the fraction of galaxies with EW(Lyα) > 25Å from ~50% at z = 6 to ~10 - 40% at z = 7 for objects around M(UV) = -20 mag. One interpretation of this downturn is that we are on the cusp of detecting the signature of reionization. However, the current small samples and poor limiting flux sensitivity produce large error bars on this measurement, which prohibits any strong conclusions regarding the timing of reionization.

The wide-field spectrographs on GMT (GMACS and NIRMOS) will provide the collecting area and large fields-of-view needed to sample large numbers galaxies with faint Lyα emission. An optical spectrograph such as GMACS offers the technological advantage of its flux sensitivity out to the silicon cut off (> 1 micron) and large field of view to measure this signature of reionization out to z < 7.4 (NIRMOS will extend this to higher redshift with a smaller field of view). Both instruments will provide spectral resolution sufficient to avoid contamination by sky lines. Figure 7.7 shows that, with R ~ 2000, a substantial portion of the spectrum is available to detect galaxies with Lyα emission as low as equivalent width (Wλ) > 10 - 20Å in a 2-hour integration with GMT/GMACS. Similar results are expected with NIRMOS. Finally, the simultaneous red and blue channels of GMACS also allow one to reject confusing sources (e.g. other lines) with high efficiency.

Figure 7.7 Sensitivity limits to Lyα emission EW (in units of angstroms) for galaxies with m = 26 and 27 (AB mag) as indicated, adapted from Schenker et al. (2011) with a spectral resolution of R ~ 2000. The sensitivity limits correspond to a 12h exposure with Keck/LRIS, which is comparable to a 2h exposure with GMT/GMACS.

Current limits from 8 - 10m telescopes suggest that the fraction of Lyα emission at z = 7 drops to 10%. Any survey with the goal of measuring this fraction accurately (to within 20%) requires the detection of a minimum of 30 LBGs with Lyα emission at z = 7. This is an order of magnitude increase in the known number of such galaxies. Acquiring these numbers will be challenging since current photometric searches suggest there is approximately one z = 7 candidate galaxy per 10 square arcmin (Finkelstein et al. 2010).

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If 10% of these show Lyα emission with EW > 25Å, a survey must target ~300 candidate z = 7 sources. The surface density of z = 7 candidates implies there will be 14 - 15 objects per mask for an instrument like GMACS on the GMT, so targeting ~300 z = 7 sources will require 20 spectroscopic masks, covering a total field of view of 0.8 sq. deg. -18 2 A 2-hour GMACS observation will reach a limiting line flux of 2 - 3 x 10 erg/sec/cm (Figure 7.7), comparable to the measured line flux in the few confirmed z = 7 sources, which required integrations over 5 times longer (e.g., Fontana et al. 2011). Assuming two hours of integration per mask, the survey envisioned here would require 40 hours without overheads. The GMT with its large A spectroscopic capability is uniquely suited to this program.

7.6 Synergy with 21cm Tomography

Currently, a major undertaking in radio astronomy is the construction of a number of arrays designed to peer back into the epoch of reionization by detecting the redshifted 21cm fine structure line from neutral hydrogen. This emission will be detectable at a number of redshifts, providing a three-dimensional tomographic map of the structure of the neutral IGM out to the highest redshifts. With their large collecting areas, the Square Kilometer Array (SKA) and its path-finders (e.g. MWA, ASCAP, PAPER) will probe the epoch of reionization with extremely high sensitivity. The SKA will be built in the southern hemisphere (dually situated in South Africa and Australia), and thus will provide a great opportunity for synergy with the GMT. Combining a GMT study of the clustering of Lyα emitting galaxies with 21cm observations from the SKA (Figure 7.8) will allow us map both the neutral and ionized gas allowing detailed studies into the state of the IGM during the epoch of reionization.

Figure 7.8 A simulation of the HI brightness temperature at z = 12.1 (left), 9.2 (middle) and 7.6 (right) from Furlanetto et al. (2004). The Square Kilometer Array will perform similar measurements, mapping the neutral gas out to z > 10. Pairing these observations with GMT/NIRMOS observations of Ly emission from galaxies at the same redshifts will map out both the neutral and ionized gas in the same volume.

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7.7 High Resolution Spectroscopy of Quasars

While redshifted 21cm studies offer the hope of seeing the IGM in emission, light from distant ultraviolet sources allows us to probe the IGM in absorption. Lyα absorption from over-dense regions of the IGM imprints a forest of dense features on the spectra of distant quasars. As the Universe transitions from ionized to neutral with increasing look-back time, the “forest” becomes a deep opaque trough in the spectra of quasars and gamma-ray bursts (GRBs) at wavelengths below Lyα in the rest-frame of the object (Fan et al. 2006; Totani et al. 2006). Detailed studies of intergalactic absorption lines enable measurements of quantities such as the IGM temperature (Bolton et al. 2010) and metal abundances (Ryan-Weber et al. 2009), as well as the ionization state of the IGM in the early Universe. These data contain valuable information on the reionization process that is inaccessible with other, complementary observational probes of the reionization era.

Figure 7.9 Synthetic Ly absorption spectra of a quasar at z = 6.15, shown with the resolution and signal to noise typical of spectra obtained on 8-m class telescopes (upper), and (lower) as it would look with GMT using a high resolution (R ~ 40,000) optical spectrograph such as G-CLEF.

Useful observations with 8 - 10m apertures require very long integration times (e.g. >10 hours) and the use of spectral resolutions (R ~ 3000) that are not well suited to determinations of the physical state of the IGM at these critical early times. The leap forward in sensitivity provided by the GMT, combined with high-resolution optical spectroscopy, will provide the capability to probe the IGM in the cosmic dawn era at z = 5 - 7 at high resolution and superior signal-to-noise.

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Figure 7.9 displays detailed simulations of intergalactic Lyα absorption in the spectrum of a z = 6.15 quasar. The spectra are constructed using cosmological hydrodynamical structure formation simulations combined with a line of sight radiative transfer model for the ionizing radiation field produced by the quasar. The upper panel shows the normalized transmission blue-ward of the Ly emission line against observed wavelength. The spectrum has resolution R = 2800 and a S/N = 20 per 3.0Å/pixel, representative of moderate resolution and signal-to-noise data obtained with Keck/ESI for the brightest known quasars (i ~ 22) at z ~ 6 (Fan et al. 2006). The lower panel shows the same spectrum, but for R = 40000 and S/N = 30 per 0.2Å/pixel, representative of the improvement in resolution and signal-to-noise attainable with a high-resolution optical spectrograph on the GMT such as G-CLEF (in an 8-hour integration). The much higher spectral resolution enables the intrinsic widths of Ly absorption lines in the quasar proximity zone (orange shading) to be fully resolved, allowing their thermal widths to be measured; the thermal state of the IGM provides a valuable indirect probe of reionization. Narrow features indicating a highly ionized IGM (and which are difficult to observe in the trough of absorption in the lower resolution spectrum) are clearly apparent in the high resolution simulation (cyan shading). The statistics of these regions may be used to place constraints on the reionization history and the ionization state of the IGM (Gallerani et al. 2008). Furthermore, the overall shape of the Lyα transmission close to the quasar rest frame is well-resolved; this may be used to identify the possible damping-wing signature of neutral gas along the quasar or GRB sight-line (Miralda-Escude 1998).

Access (at lower signal-to-noise) to spectra of fainter objects at redshifts up to z = 7 will result in larger data sets and in a commensurate improvement in the statistical power of the data. Existing data have limited discriminatory power because of the large scatter in individual measurements due, in turn, to variations in the IGM density and ionization state along individual sight-lines (Mesinger 2010). Larger data sets will significantly benefit analyses of the IGM neutral hydrogen fraction using Lyman series absorption. The sizes of the windows of transmission around high redshift quasars, which arise due to the enhancement in the ionizing radiation around these sources, can provide a useful probe of the ionization state of the IGM. The relative sizes of these regions in Lyα and Lyβ absorption should provide a constraint on the IGM neutral fraction at z > 6. A sample of tens of spectra at moderate resolution and signal-to-noise ratio are needed to provide a clean measurement (Bolton & Haehnelt 2007). With sufficiently large data sets the frequency of dark gaps in absorption spectra may also be used to constrain the IGM neutral fraction at z > 6. The ability of the GMT to obtain such high quality data sets will provide a significant step forward in our understanding of the state of the IGM in the cosmic dawn epoch.

The high-resolution NIR spectroscopic capability afforded by GMTNIRS will allow these studies to be extended to z > 7. Combining the high spectral resolution with adaptive optics (AO) correction will improve the sensitivity for IGM studies. Though few QSOs at such high redshifts are currently known, the number may grow substantially as new wide- field IR surveys are completed (e.g. UKIDSS, VISTA). However, current studies of the evolution of the AGN space density imply that they are increasingly rare at higher redshift (e.g., Richards et al. 2006).

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Gamma ray bursts, on the other hand, should be increasingly more common at high redshift (e.g., Tanvir et al. 2009; Cucchiara et al. 2011), especially near the epoch of the first galaxies where more massive stars may be more common. GRBs may thus be better suited as beacons through the IGM to the highest redshifts. High resolution NIR spectroscopy of z > 7 QSOs or GRBs will allow study of the ionization state of the IGM well into the epoch of reionization. The high sensitivity afforded by the GMT will allow measurements to be made of metal absorption lines, which will provide useful information on the ionization state at a much higher neutral fraction than possible with the Gunn-Peterson measurements (e.g., Oh 2002).

7.8 Summary

Exploring the epoch of reionization of the IGM is a key goal for astrophysics in the next decade. The GMT can make unique contributions to probing galaxies in the first billion years after the Big Bang. Working in tandem with the Webb Telescope and the SKA/pathfinders, the GMT will probe the properties of the IGM through studies of Lyα emission in faint galaxies and Lyα absorption along sight-lines to distant quasars and gamma-ray bursts. Understanding the evolution of the physical state of the gas between galaxies in the early Universe is vital to a complete picture of the formation of galaxies and the recycling of baryons.

References

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Pawlik, A. H., Milisavljevic, M., Bromm, V. 2011, ApJ, 731, 54 Nagao, T. et al. 2008, ApJ, 680, 100 Richards, G. T., et al. 2006, AJ, 131, 49 Robertson, B., et al. 2010, Nature, 468, 49 Ryan-Weber, E., Pettini, M., Madau, P., Zych, B., 2009, MNRAS, 395, 1476 Scannapieco, E., Schneider, R., Ferrara, A., 2003, ApJ, 589, 35 Schenker et al. (2011), astro-ph: 1107.1261 Tanvir, N. R. et al. 2009, Nature, 461, 1254 Tumlinson, J., Shull, M. J., 2000, ApJ, 528, 65 Vanzella, E., et al. 2011, ApJ, 730, 35 Wang, J.-X., Malhotra, S., Rhoads, J., Zhang, H., Finkelstein, S. L., 2009, ApJ, 706, 762

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8 Transient Phenomena Hsiao-Wen Chen, J. Craig Wheeler

8.1 Introduction

Investigating transient phenomena (those with variability time-scales ranging from a few minutes to a few months) is a new frontier in astrophysical research. They represent a vast, unexplored parameter space for testing fundamental physics in powerful cosmic explosions such as γ-ray bursts (GRBs) and supernovae (SNe). Various synoptic all-sky surveys, such as Pan-STARRS and LSST, are designed to study the dynamic Universe by documenting new transient events, but a physical understanding of the origin and nature of these transient phenomena requires spectroscopic follow-up with large telescopes such as the GMT.

8.2 Optical Transients of Long-Duration Gamma-Ray Bursts

GRBs are among the most energetic events in the Universe. In particular, long-duration GRBs are believed to originate in the catastrophic deaths of massive stars (see Woosley & Bloom 2006 for a review). Some bursts are followed by optical afterglows (e.g. Akerlof et. al 1999; Kann et al. 2007; Bloom et. al 2009) that can briefly exceed the absolute brightness of any known quasar by orders of magnitude (Figure 8.1). Similar to high redshift quasars (QSOs), GRB afterglows can serve as a sensitive probe of “dark” intervening gas that is either local or external to the burst progenitor’s environment. Since GRB afterglows rapidly decline however, an effective exploitation of these lighthouses requires rapid follow-up spectroscopy (within 2 days) with 8m-class telescopes.

Figure 8.1 Rest-frame optical light curves of luminous GRB afterglows and SN2006gy (Smith et al. 2007), compared to rest-frame, absolute r-band magnitudes of known QSOs from SDSS (Schneider et al. 2007; the grey horizontal band). The most luminous QSO to date is marked by the dashed horizontal line. The brightest GRB afterglow recorded was GRB 080319B. At early times, the optical transient was ~103 times more luminous than the most luminous QSO (Bloom et al. 2009).

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GRB afterglows are suitable cosmic probes because a large fraction (>50%) are known to originate at redshift z > 2, including a growing fraction at z > 6 when the Universe was less than one billion years old. The redshift distributions of known GRBs and QSOs in Figure 8.2 shows that GRB afterglows are as good as QSOs for probing the z = 2 - 4 Universe, and better for probing the re-ionization epoch at z > 6. Rapid echelle or moderate-resolution spectroscopy of the afterglows is critical both for constraining the distances (and therefore the energy output) of individual burst events, and for probing the physical conditions of gaseous clouds along the lines of sight.

Figure 8.2 Redshift distribution of 219 GRB afterglows identified as of Nov. 2010 (red solid curve), compared to the redshift distribution of i < 20 QSOs found in the SDSS (black dash-dotted curve). The thin curves show their corresponding cumulative redshift distribution. GRB afterglows are as good as QSOs for probing the z = 2 - 4 Universe and better for probing the re-ionization epoch.

In contrast to a QSO spectrum, which exhibits broad emission features and complex bumps and wiggles that are not fully understood, the continuum spectrum of an optical afterglow is remarkably simple and is well characterized by a power-law function. For example, Figure 8.3 shows the afterglow spectrum of GRB050730 at z = 3.968 obtained using MIKE on the Magellan Clay Telescope (Chen et al. 2005). Blue-ward of the host damped Ly absorber (DLA) at 6040Å is a forest of Ly absorption lines from foreground gaseous clouds, including a classical DLA at z = 3.564. These absorption features reveal the line-of-sight matter density fluctuations that are otherwise invisible in traditional galaxy surveys, and the echelle resolution (FWHM of the spectral resolution = 10km/s) further allows accurate empirical constraints for the ionization state, kinematics, and metallicity of these intervening clouds (e.g. Chen et al. 2005; Prochaska et al. 2007). The absence of flux below 4530Å indicates that little Lyman continuum photons escape the host galaxy.

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Red-ward of the host DLA are numerous narrow absorption features of neutral and low- ionization species in the interstellar medium of the GRB host, including excited transitions such as FeII. These excited ions were seen for the first time in a distant star- forming galaxy, and the time variation of their abundances allows us to constrain the distance of the absorbing gas from the luminous GRB afterglow. A subset of absorption features produced by different foreground gaseous clouds (color-coded by redshift) along the line of sight are also marked in the figure to demonstrate the feasibility of applying GRB afterglows as cosmic probes for studying the interstellar and intergalactic media at -1.88 high redshift. The continuum is best described by a power-law model ƒ (λ) α λ (the magenta line).

Figure 8.3 The afterglow spectrum of GRB050730 at z = 3.968 obtained using MIKE on the Magellan Clay Telescope (Chen et al. 2005). See Section 8.2 for a description of the wealth of information that can be obtained from a spectrum of this quality.

The simple power-law shape of an afterglow spectrum (see also Barth et al. 2003) offers additional advantages for probing the gas content in the distant Universe, including robust measurements of dust extinction laws and gas surface density. Figure 8.3 demonstrates the feasibility of applying afterglows as cosmic probes for studying the interstellar and intergalactic media at high redshift. The visible and near-IR spectrographs planned for GMT are ideally suited to these studies.

Prompt localization of GRB afterglows and rapid responses of optical telescopes are critical to obtaining high-resolution spectroscopy during their brief periods of extreme brightness. Current or planned space- and ground-based facilities are capable of rapidly

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identifying new GRB afterglows over the next decade and beyond. For example, the current generation gamma-ray satellite, Swift, launched in November 2004, can provide nearly instant localization of new GRBs (Gehrels et al. 2004). In the past five years, Swift has detected over 500 new bursts, roughly 85% of which were instantly localized using its on-board x-ray telescope, or with optical-IR telescopes on the ground. The satellite is expected to continue the operation through 2015, at which time the Space Variable Objects Monitor (SVOM) (a Chinese-French space mission) is expected to be launched (Paul et al. 2008). Similar to Swift, SVOM is also designed to deliver rapid localization of new GRBs for ground-based follow-up.

Other proposed space missions include the Joint Astrophysics Nascent Universe Satellite (JANUS; Fox et al. 2010) and the Energetic X-ray Imaging Survey Telescope (EXIST; Grindlay et al. 2010). These new satellites will deliver new, localized GRBs well into the decade beyond the Swift era.

Figure 8.4 Expected NIR K-band light curves of GRB 080319B at redshifts from z = 0.4 to 16 (Bloom et al. 2009). A transient at z = 10, as luminous as the afterglow of GRB 080319B, would still have K ≈ 13 mag 15min after the initial trigger. However, GRB 080319B has by far the most luminous optical afterglow detected, and there exists a large scatter in the peak magnitudes and light curve shapes of these optical transients. The inset shows the optical light curves of a large sample of previously identified GRB afterglows that have been shifted to a common redshift of z = 1.

Once a GRB is detected by one of these instruments, rapid follow-up observations from available ground based telescopes are vital. But the windows of opportunity for making these observations are short. The required telescope response timescales can be estimated based on known afterglow light curves. Figure 8.4 shows the expected K-band light curves of GRB080319B at different redshifts from z = 0.4 to z = 16 (Bloom et al. 2009).

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The curves exhibit a scatter of more than 10 mag in the initial brightness. For a power- -0.4m -α law model, m0 and alpha in m(t) = -2.5log (10 0 t ), the decay index varies between α ≈ -0.5 and α = -2 (Zeh et al. 2006). At high redshifts, the afterglow will have a fainter observed magnitude but the declining time scale is stretched out by a factor of (1 + z).

Applying the observed scatter to the expected K-band light curves of distant GRB afterglows, we estimate a median brightness of ~18 mag, 15 minutes after the initial burst for sources at z ≈ 10. For a source of K = 18 mag, it would take about 1 hour of integration time using FIRE on the Magellan Clay telescope to reach S/N ≈ 15 at a spectral resolution of R = 6000. But since the optical transient fades with time, the actual S/N achieved in an hour would be less: For a source of initial brightness K = 18 and a fiducial decay index of α = -1, we estimate that S/N ≈ 6 can be reached in an hour with the GMT. While increasing the exposure time will yield better S/N, the gain in the signal is less than 50%. Better S/N can be reached with faster telescope response (to catch the optical transient at brighter magnitudes) or lower-resolution IR spectrographs.

A particularly important utility of GRB afterglows is probing the infant Universe using NIR echelle spectroscopy for addressing a number of fundamental issues including:

• What are the initial conditions for the formation of the first-generation stars? • How do the reionization and chemical enrichment of the intergalactic medium progress with time? • What are the sources of reionization?

8.2.1 The First Stars

The first stars (Population III) are expected to be massive (>100 MSun) and form out of metal-free gas (see e.g. Bromm & Larson 2004). They are promising candidates for reionizing the intergalactic medium and for polluting the intergalactic medium with heavy elements during the reionization epoch. While Pop III stars may contribute as much as 5% of the total star formation rate density at z ≈ 10 (Bromm & Loeb 2006), (see Figure 8.5 left panel), to date no robust detection of Pop III stars has been made.

The origin of long-duration GRBs in the final evolving stage of massive stars implies that these afterglows are the signposts of extreme star-forming regions at all epochs. For a -9 constant GRB efficiency of η ≈ 2 x 10 bursts/MSun, theoretical models predict that at z ≈ 10 Pop III stars may contribute to 5% of the GRB population (right panel of Figure 8.5). Afterglow spectra record physical properties (ionization, temperature, gas-phase metallicity, and kinematics) of the progenitor environment through numerous absorption features (e.g. Figure 8.3), providing key constraints for the formation and evolution of early-generation stars.

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Figure 8.5 Contribution to the cosmic star formation rate density from Population I/II and Population III stars in the left panel and predicted GRB rate as a function of redshift in the right panel for a constant GRB efficiency of GRB 2 x 10-9 bursts per unit solar mass (from Bromm & Loeb (2006)). Both calculations assume that the Universe was reionized at z ~ 17. The dotted curves represent weak chemical feedback, and the dashed curves represent strong chemical feedback.

8.2.2 The Reionization Epoch

An accurate determination of reionization history can greatly improve the accuracy and precision with which fundamental cosmological parameters can be derived (e.g. Knox et al. 1998). Whether the reionization of the intergalactic medium proceeded rapidly or over an extended period of time is uncertain (c.f. Fan et al. 2006; Becker et al. 2007). While theoretical models favor a rapid reionization process of the intergalactic medium, CMB polarization data show that instantaneous reionization at z < 6.7 is rejected at the 3σ level (Dunkley et al. 2009; Larson et al. 2010).

Empirical studies of the reionization epoch have focused primarily on observing the most 9 distant QSOs at z > 6. However, QSOs are powered by supermassive black holes in ~10 MSun dark matter halos, which form presumably through mergers of smaller halos under the ΛCDM hierarchical formation paradigm (e.g. Haiman & Loeb 2001) and are substantially less common at earlier times. In contrast, the first stars are expected to form 6 in ~10 MSun halos (e.g. Barkana & Loeb 2001), which are already present at z > 7. Since these stars subsequently generate GRBs, GRB afterglows can serve as a more abundant and sensitive probe of the intergalactic medium during the epoch of reionization.

In addition, Madau et al. (1999) have shown that the Universe may be reionized by stellar sources rather than QSOs. Under this scenario, the onset of the reionization would mark the formation epoch of the first generation stars. Depending on the volume density of the ionizing sources and the mass loss rate of massive stars, they may also explain the early enrichment of the intergalactic medium.

Observations of z > 6 GRBs (e.g. Kawai et al. 2006; Tanvir et al. 2009; Salvaterra et al. 2009) help to unveil the sequence in which the Universe became reionized (Mesinger et al. 2004; 2005). Figure 8.6 shows the afterglow spectrum of a GRB at z = 6.295. In

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addition to strong and narrow metal-line absorption features that allow us to determine the redshift of the source, a damping trough is observed at 8900Å. Interpreting the red damping wing as entirely due to the interstellar medium of the GRB host leads to a gas 21 2 surface density of N(HI) ~ 4 x 10 /cm (the solid curve and inset of Figure 8.6) in the host medium. Such high density is comparable to what is seen in the denser part of star- forming regions in the Milky Way. However, a neutral intergalactic medium is also expected to contribute to the absorption trough (e.g. Miralda-Escude 1998).

Figure 8.6 A combined afterglow spectrum of GRB 050904 at z = 6.295 (Kawai et al. 2006). The data were taken 3.4d after the initial burst using the Faint Object Camera and Spectrograph (FOCAS) on the 8m Subaru Telescope with a total exposure time of 4 hours and a spectral resolution of FWHM ~ 8.5Å at ~ 9000Å (Totani et al. 2006). The strong, narrow absorption features due to O0, Si+, and C+ allow an accurate redshift measurement of the host galaxy. Interpreting the damping trough at 8900Å as entirely due to the host ISM (the red solid curve; see also the inset), Totani et al. (2006) estimate a host-DLA of log N(HI) ~21.6. Analyzing the red damping wing also allows us to constrain the neutral fraction of the IGM. Different curves show different model expectations (see text).

The red damping wing therefore offers a unique avenue for constraining the neutral fraction of the intergalactic medium at this early time. We illustrate the methodology in Figure 8.6 where we compare absorption line wings due to 100% neutral intergalactic medium plus host interstellar hydrogen with column densities that range from log N(HI) = 21.3 to 21.6. The high column density model overproduces the absorption at the red wing, while the lower column model underestimates the absorption at the core. Comparisons between observations and different model predictions indicate that the intergalactic medium was already partially ionized by z = 6.3. Such studies are only possible with rapid, early-time afterglow spectroscopy with a large aperture telescope.

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8.3 Supernovae

The frontier for supernova (SN) research in the GMT era will be routine, temporally well sampled, optical and NIR spectropolarimetry to address key questions regarding the nature of the progenitors and explosion mechanisms. Answers to these questions bear significantly on studies of dark energy (Riess et al. 1998; Perlmutter et al. 1999).

About 1 in every 1000 galaxies will have a live supernova in it. Efforts should begin now to construct an efficient pipeline to deconvolve SN spectra from galaxy spectra and hence to discover and identify supernovae from spectra alone. The rate of discovery of supernovae promises to expand dramatically with the Palomar Transient Factory and Pan-STARRS. The LSST alone is anticipated to discover more than 100,000 supernovae per year (Wood-Vasey et al. 2011). One in a thousand of these (or about 100 per year) may also be strongly lensed (Oguri & Marshall 2010). An important task will be to decide which of this plethora of events should be studied in more detail with spectroscopy on a 20m class telescope like the GMT.

8.3.1 Type Ia Supernovae

Among the major outstanding issues in the study of SN Ia is proof that they arise in binary systems, and, if so, in single degenerate systems, double degenerate systems, or some mix of the two. Clues to the progenitor system may arise by the detection of circumstellar matter. Search for and detection of variable Na D absorption in the early spectra of SN Ia is currently a topic of great interest (Simon et al. 2009).

Another important clue has come from the discovery and study of high-velocity Ca and Si features moving at 20,000 - 30,000km/sec, as shown in Figure 8.7. The high-velocity calcium lines may arise from gas at solar abundance associated with some circumstellar medium, but the high-velocity silicon must represent freshly synthesized matter from the explosion (Marion et al. 2011, in preparation). The observed kinematics may be associated with the collision of the ejecta with a shell of about 0.02 solar masses that lies 15 at substantially less than 10 cm in order not to contaminate the rising light curve (Gerardy et al. 2004). No such collisionally induced luminosity is seen (Hayden et al. 2010). These high-velocity features are highly polarized. Similar features are also seen in Type Ib/c supernovae.

Observing these features early, i.e. well before maximum, is critical. Observing at larger redshifts with the associated time-delay may make this task somewhat less challenging and LSST should find many SN Ia very early after explosion. These features are under close study now, but the sample remains small and there will remain much to do in the GMT era.

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Figure 8.7 Time series of low-resolution spectra of Type Ia supernova SN 2010kg obtained with the Hobby-Eberly Telescope showing the evolution of the high-velocity CaII infra-red triplet. Note the especially broad feature in the top spectrum (1202) that may be blended with OI 7774 and the evolution of this feature to lower velocities (G. H. Marion, unpublished). Observing these features well before the peak magnitude is critical. LSST should find many SN Ia very early after explosion.

Important questions concerning the nature of SN Ia explosions remain unanswered. Observations, especially the spectral evolution from the UV to the NIR, strongly suggest that the explosions result from the transition from a subsonic deflagration to a supersonic detonation in a carbon-oxygen white dwarf of nearly the Chandrasekhar mass. New developments in the theory of this combustion transition are imminent, but detailed comparisons between observations and simulations will be needed, and the GMT will be able to play a significant role in this endeavor. Another enticing idea is that all SN Ia may have a nearly standard but asymmetric shape like a standard “egg” observed from different aspects (Maeda et al. 2010; Maund et al. 2010). Further development of these ideas would be aided by spectropolarimetry in the visible and near-IR on the GMT.

Rest-frame near-infrared spectroscopy is an important tool for studying the outer layers of SN Ia. The initial composition of the progenitors is carbon and oxygen, so those elements and their burning products are key targets of spectroscopy. Carbon burning occurs at relatively low densities and temperatures, producing O and Mg. Carbon and oxygen do not produce strong lines in the optical band, and Mg, which can, tends to be

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heavily blended. These elements do, however, have strong lines in the NIR. Carbon is not detected down to very tight limits in most “normally-bright” supernovae, although it is detected in subluminous SN Ia (Marion et al. 2006). Since O and Mg are found in the same range of velocity and hence physical space, given homologous expansion, the absence of C and the presence of its burning products is strong evidence that a detonation must have formed to catch up with and burn the supersonically expanding outer layers.

Figure 8.8 shows that the Mg, Si, S, and Ca are significantly polarized in the outer layers, whereas O is not. These asymmetries in chemical distributions in the outer layers are another important clue to the nature of the explosion.

Figure 8.8 Spectropolarimetry of the “core normal” Type Ia supernova SN 2001el showing the total flux spectrum (top) and the polarization spectrum (bottom). Note the significant polarization in the Si and especially in the high-velocity Ca lines (Wang et al. 2003; Kasen et al. 2003). Asymmetries in chemical distributions in the outer layers are another important clue to the nature of the explosion.

8.3.2 Core-Collapse Supernovae

For core-collapse supernovae, the pre-supernova evolution is understood only qualitatively, while the mechanism of explosion remains elusive. Neutrinos must play a large role, if only to sap the energy derived from the collapse. Polarization studies have established that asymmetries are common and associated with the deep interior of the explosion (Wang & Wheeler 2008). These non-spherical shapes are not necessarily axisymmetric. Polarization studies also suggest that the breakout shocks are likely to be asymmetric, a significant factor in analyzing breakout dynamics (Couch et al. 2009, 2011). The controlling timescale of breakout is likely to be set by the lateral propagation

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of shocks around the periphery of the progenitor, not the light travel time as is often assumed. These asymmetries may be the result of the asymmetric deposition of neutrino heating, but suggest that a role for magneto-rotationally driven jets is worth considering.

We know that the collapse of massive stars is linked to long-duration GRBs, as outlined earlier. In the context of the underlying SNe, we would like to know what distinguishes a “broad-line” from a “normal” Type Ic supernova and what fraction of broad-line SN Ic are associated with GRBs, and why? NIR spectroscopy will be critical to the pursuit of such questions, for instance to readily reveal the presence or absence of He in the spectrum. With its capacity for deep optical and NIR spectroscopy and spectropolarimetry, the GMT will contribute greatly to the study of core-collapse supernovae.

8.3.3 Super-Luminous Supernovae

The recently delineated category of super-luminous supernovae revealed by the Texas Supernova Survey (TSS) (Smith et al. 2007; Quimby et al. 2007, 2011; Chatzopoulos et al. 2011) is likely to remain a category of great interest in the GMT era. The TSS (now continuing as the ROTSE Supernova Verification Project, RSVP) used the robotic ROTSE telescope to conduct a nearly unbiased search for supernovae over a volume of 9 3 about 10 Mpc over a 5-year period. The result was the discovery of SN 2005ap (z = 0.2832), SN 2006gy (z = 0.01919), SN 2006tf (z = 0.074), SN 2008es (z = 0.205), and SN 2008am (z = 0.2338), still a significant fraction of all published SLSNe.

Many SLSNe are thought to result from the collision of the explosion with dense, optically-thick circumstellar media. They may be related to luminous blue variables. The thick shell obscures the explosion, leaving its nature, even its mass and energy rather uncertain. The progenitor mass-ejection process also remains uncertain. Others of this general category may be the result of electron positron pair instability, as is expected theoretically to occur in very massive stars.

These rare but bright events represent a challenge to account for their light curve shapes and amplitudes (see examples in Figure 8.9), and their spectral evolution. They have proven to be a rather heterogeneous collection of events. Some show strong emission lines of hydrogen, making them a variant of Type IIn supernovae. Among these are: SN 2006gy, SN 2006tf, SN 2008am, SN 2008fz, and SN 2008iy. Some show a continuum near maximum, and hydrogen only later with a linear (in magnitudes) decline of their light curves, an example being SN 2008es. Some may show no hydrogen at all, for example: SN 2005ap, SCP06F6 (z = 1.189), PTF09cwl (z = 0.349), PTF09cnd (z = 0.258), and PTF09atu (z = 0.501). These latter events show little or no evidence of circumstellar interaction, yet radioactive decay of 56Ni does not provide sufficient energy. New insights into this latter category of SNe have been presented by Quimby et al. (2011). Lastly, SN 2007bi (z = 0.1279) has no hydrogen and no sign of circumstellar interaction. Its behavior is consistent with radioactive decay, so this event could result from pair instability (Gal-Yam et al. 2009).

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We need to characterize and understand this diversity; a large collecting area coupled with efficient spectrographs and, perhaps, polarimeters could provide the needed breakthroughs.

Figure 8.9 Left: Light curves of a sample of super-luminous SNe (from Gezari et al. 2008). Right: Light Curve of super-luminous SN 2006gy in contrast to other supernovae (Smith et al. 2007).

8.3.4 Supernova Rates

GMACS on the GMT will achieve a S/N ratio of about 10 in an hour-long exposure at a th resolution of R = 2000 down to about 24 magnitude. The performance of NIRMOS is expected to be comparable. Using the cosmology simulators of Ned Wright and Chip Kobulniky with H0 = 71 km/s/Mpc, Ωm = 0.27, and Ωλ = 0.73, this means that a Type Ia supernova with a peak absolute magnitude of -19 can be observed to a distance modulus of about 43 and a redshift of z ~ 0.7. SLSNe have a peak absolute magnitude of about -22 and hence can be observed to a distance modulus of about 46, or z ~ 2.0. The breakout shock of a Type II plateau might be about -17 in the NUV (Tominaga et al. 2010) and could be observed to a distance modulus of about 41 and hence z ~ 0.3.

Taking SN Ia as a reference point, a redshift of z ~ 0.7 corresponds to a co-moving volume of about 70 Gpc3. With a rate of SN Ia of about 3 x 10-5/Mpc3/yr (Dilday et al. 6 2010), SN Ia would occur at the rate of about 2 x 10 /yr within that volume. The probability that a SN Ia is near peak light is ~ 0.01, so in half the sky the GMT could, in 4 principle, observe ~10 SN Ia per year near peak light at z ~ 0.7. In practice, LSST will not find all the supernovae, so the actual rates would need to be scaled for that and other inefficiencies and for the fact that some of the science goals would be best achieved prior to or after maximum light when the supernova was dimmer.

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Occurring at a rate of about 3 x 10-7/Mpc3/yr (Quimby et al. 2009), SLSNe are a factor of 100 more rare than SN Ia, but can be observed to about twice the distance and about 10 times the volume. SLSNe should therefore be minimally observable near maximum (at a redshift of ~2) with the GMT at the rate of about one per 10 minimally-observable SN Ia. With a rate of about 10-3/Mpc3/yr, minimally observable breakout events would be about 10 times more frequent than minimally-observable SN Ia, but observable in approximately one tenth the volume, so the limiting rate might be roughly comparable to that of minimally observable SN Ia. These latter comparisons ignore variations in the probability of observing near maximum.

For all these events (and more) it would be desirable to get pre- and post-maximum spectra and to acquire spectropolarimetry that requires multiple exposures. These factors will reduce the effective rates at which these events are observable. In addition, all these rates may depend sensitively on the both the redshift and the band in which the observations are made. These topics merit more quantitative study. The science goals would be to (1) conduct spectral evolution studies, (2) conduct line-of-sight studies (with high resolution; the complement to the techniques employed with GRBs as discussed above), and (3) study the correlation with the star formation rate and with host galaxy properties. Large survey programs (e.g. LSST) can provide the needed samples and GMT can provide spectra at both early and late times to characterize the populations and probe physical conditions in the ejecta.

8.4 Other Transient Sources

In addition to long-duration GRBs and supernovae, other known transient events include short-duration GRBs and flares near galactic nuclei. Little is known about these phenomena, and significant progress is expected to occur during the GMT era with its imaging and spectroscopic follow-up capabilities.

Unlike long-duration GRBs, which often display bright x-ray and optical afterglows, only a few short-duration GRBs have been detected in afterglow radiation in x-ray and optical that would allow accurate localization of these sources. While the origin of long-duration GRBs in the death of massive stars is definitively established by their association with core-collapse SNe, searches for supernova events associated with short bursts have yielded null results (Fox et al. 2005; Hjorth et al. 2005) and some of the few localized short-duration GRBs are found near old stellar populations (e.g. Bloom et al. 2006).

To explain the short burst duration and lack of supernova association, a leading model of short GRB progenitors is binary merger, either neutron-star/neutron-star or neutron- star/black hole. This model can be tested by combining gravitational wave observations and afterglow follow-up. In a binary coalescence, the energy and angular momentum are carried away by gravitational waves. A direct association of gravitational wave detections and afterglow radiation of short GRBs therefore provides a critical test for the binary merger progenitor model (e.g. Lee & Ramirez-Ruiz 2007). Spectroscopic follow-up of faint afterglows associated with short GRBs is also necessary to unambiguously establish the distances of these sources and to study the progenitor environment.

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Finally, a large fraction of galaxies are believed to host supermassive black holes at their centers. Mass deposits onto the central black hole due to tidal disruption of surrounding gaseous clouds and stars give rise to near-infrared flares near galactic centers (e.g. Genzel et al. 2003). Observations of these flares provide a unique window for studying the size and spin of supermassive black holes outside of our own Milky Way.

8.5 Summary

In the past decade new classes of transient sources have become increasingly important to astrophysics, both as intrinsic phenomena and as probes of intervening material. The coming decade, with its emphasis on large-area time-domain surveys promises to reveal new classes of transient phenomena as well as larger sample of SNe, GRB afterglows and other transients. The GMT’s spectroscopic capabilities will offer unique opportunities to exploit these survey databases to probe the early Universe and the most energetic phenomena known.

Furthering our understanding of Type Ia supernovae will require early detection combined with optical and NIR spectroscopy and spectropolarimetry. More work can also be done on the late-time nebular phase that is relatively understudied because supernovae are naturally faint at that phase. The goals for this line of research would be to better understand the progenitor evolution, the explosion mechanism, and control of systematics when using Type Ia SNe as cosmological probes.

Studying core-collapse supernovae also requires optical and NIR spectroscopy and spectropolarimetry with the goals here being understanding the mechanism, the origin of asymmetries, the physics of breakout, and the link to GRBs.

Super-luminous supernovae are of particular interest: They are rare and heterogeneous; they tend to occur more frequently at high redshift and at low metallicity (Neill et al. 2011); and their spectra, their spectral evolution, and the shape and amplitude of their light curves, challenge classical SNe models. Some SLSNe may be related to pair instability, but pair-instability models need more exploration in full 3D hydrodynamic simulations including radiative transfer. Given their great luminosity, they may also make valuable new tools to study cosmology.

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9 Scientific Synergies with Other Major Facilities Patrick McCarthy

9.1 Introduction

Astronomical facilities rarely work in isolation and most problems in contemporary astronomy and astrophysics are approached with a variety of observational and theoretical tools. The GMT and other ELTs will have unique capabilities, but input from other sources will maximize their impact. Similarly, a number of exciting new ground- and space-based facilities are on the horizon and these too will benefit from spectroscopic follow-up with large apertures and ELTs in particular.

The center of mass for ground-based astronomy is moving to the southern hemisphere and this makes the case for synergy between the GMT and facilities such as ALMA, LSST, and SKA all the more compelling.

Table 9.1 Top level science areas

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First light on the GMT and other ELTs is 8 - 10 years distant. For the present discussion, we limit our time horizon to the next decade or so. Some of the missions and facilities currently under consideration will be completed within that time frame, while others will be pushed further into the future or may be abandoned or evolve into something else.

The top-level science drivers for most of the large missions or facilities under consideration are fairly similar. The US Decadal Survey and similar planning exercises in other communities around the world provide a broad vision of the scientific priorities for the coming decade, and a context in which to compare overlapping and complementary science priorities among the GMT and other facilities and missions.

In Table 9.1 we summarize the top-level science areas from the Astro2010 report alongside those from the GMT, LSST, JWST, ALMA and SKA projects. While the SKA project is still evolving, their science goals provide a useful benchmark for the SKA path- finders (e.g. MWA, ASCAP, PAPER) under development around the globe. While the precise terminology and emphasis in science area varies from one project to another, the overall correspondence is quite good. GMT and other ELTs, or the “GSMT” in particular, have a strong overlap with the Astro2010 science areas. More specialized or targeted facilities have a different balance.

9.2 Synergy with Ground-based Facilities

9.2.1 ALMA

The question of synergy between ALMA and ELTs was the topic of a three-day workshop at ESO in 2009.4 We have written a short white paper concerning synergy between GMT and ALMA and this can be found on the GMTO web site.

The GMT and ALMA science missions span a broad range of topics, from cosmology and galaxy formation to studies of star formation and the energetics of the interstellar medium. A number of strong synergies exist between the capabilities of the GMT and ALMA, and strong overlap with the key science priorities for both facilities. The southern hemisphere location of the GMT ensures that it will sample the same region of the sky as ALMA. The two facilities will probe similar physical scales: the GMT, operating at the diffraction-limit, has a resolution of 10mas at 1micron, while ALMA will achieve 10mas resolution at its highest operating frequencies.

The primary areas of scientific synergy that we have identified relate to understanding the full range of gas-phase processes in galaxies and AGN, probing star formation in the Milky Way and other galaxies, and tracing the evolution of planetary systems from the proto-planetary disk phase through to debris disks and mature planetary systems.

4 Talks can be downloaded at: http://www.eso.org/sci/meetings/almaelt2009/program.html 108 Scientific Synergies with Other Major Facilities | GMT Science Case

Figure 9.1 Herschel SPIRE image of the GOODS-North field. This three color image was made by combining deep images at 250, 350 and 500 microns. The image is confusion limited at all three wavelengths. ALMA will be able to probe the spectral properties of these 1 < z < 5 obscured galaxies with great sensitivity and angular and spectral resolution. The GMT will be able to probe the stellar content of the sources and measure they ionized gas content and abundances.

9.2.1.1 Surveys of Distant Star-forming Galaxies

ALMA will have unprecedented power to probe the early Universe and star formation in galaxies. Working in the sub-mm continuum, ALMA will be able to detect star forming galaxies at essentially any distance—the negative k-correction in the sub-mm ensures that starburst galaxies have approximately equal brightness over a very broad span of redshift. Small fields will be surveyed to extreme depths in both the sub-mm continuum and in the lines of various molecular species. The deep Herschel survey fields have revealed a high surface density of luminous obscured star forming systems at high redshifts (see Figure 9.1).

These long pointings and the Spitzer/MIPS deep surveys are confusion limited. Confusion should not be significant for deep surveys with ALMA, however. Most of the faint sub-mm/far-IR sources detected in current surveys are extremely faint in the visible, and spectroscopy of these systems pushes the current generation of optical telescopes to their limits. The GMT will allow optical/near-IR imaging and spectroscopy of the fainter and more distant star forming galaxies detected by ALMA. Modest area surveys with ALMA can be followed up with the wide-field multi-slit spectrograph on the GMT, while very deep, targeted observations are well matched to the GMT diffraction-limited AO modes.

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GMT spectroscopy of deep ALMA sub-mm surveys in legacy fields (e.g. UDF, CDFS) will resolve the full redshift distribution of the sub-mm population and address the issue of star formation at very high redshifts. Spectroscopy with the current generation of large telescopes is limited to targets brighter than R(Vega) < 25. The GMT multi-object spectrograph should reach to R ~ 27 in several hour exposures, while spectroscopy of point sources in the diffraction-limited regime could reach as faint as K(Vega) ~23 - 24.

9.2.1.2 Distant Galaxy Clusters

ALMA will have the ability to probe the structure of distant galaxy clusters on small scales using the Sunyeav-Zeldovich effect. Dedicated S-Z telescopes (e.g. South Pole Telescope, SPT) are producing catalogs of hundreds of potential high redshift clusters, all of which require some optical follow-up observations to determine their redshifts, optical richness and stellar content. These distant clusters provide an ideal laboratory for studies of stellar and chemical evolution in galaxies, and the GMT provides the required capability to explore clusters beyond the reach of current facilities. ALMA will probe mass concentrations on scales of ~15” to 2’ within clusters via high spatial resolution S-Z studies with sensitivity in the micro-Kelvin range. Kinematic studies of the constituent galaxies with the GMT multi-object spectrometer in conjunction with these deep S-Z observations will allow one to develop a detailed picture of the dynamic state of individual rich clusters. Observations of large samples of clusters, particularly over a wide range in redshift, will reveal the processes by which clusters are assembled, perhaps via the mergers of smaller groups. The GMT will be the telescope of choice for studies of these rare objects. The required fields of view are modest and the GMT optical and near- IR imaging spectrographs will be very powerful instruments for this work.

9.3 Star and Planet Formation

9.3.1 The Sub-Stellar IMF

ALMA and the GMT bring powerful tools to bear on the problem of observing the formation of stars and planets from dense molecular clouds. The GMT will detect low mass stars and substellar objects via thermal radiation from dust grains in the near and mid-IR. The great sensitivity of the GMT and its high angular resolution in the laser tomography AO mode will allow it to probe highly enshrouded protostars and protoplanets in nearby star forming complexes. While near- and mid-IR observations provide a window with greatly reduced extinction, ALMA, operating in the mm and sub- mm regions of the spectrum can penetrate even the densest protostellar cores. Jupiter- mass protoplanets with ages of a few million years will be detectable to distances of a few hundred pc with ALMA in reasonable integration times. The GMT and ALMA, by observing in different regions of the spectrum, sample a wide range of temperatures and densities and thus allow for more complete studies of star and planet forming regions in the southern sky. The nearest star forming complexes in Orion and Ophiuchus area ideal laboratories for panochromatic studies of star formation with the GMT and ALMA.

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9.3.2 Debris Disks and Protoplanets

The GMT is being designed with extreme adaptive optics and imaging of exoplanets as a high priority. Old Jupiter-class planets with small orbital semimajor axes will be detected in reflected light in the near-IR while younger giant planets will be imaged at longer wavelengths and larger distances from their parent stars. The youngest planetary sized objects are expected to be embedded in protoplanetary disks and thus may not be discernible directly against disk emission. The existence of such planets can be inferred indirectly from their clearing of regions of the disk. With the GMT, high-resolution mid- IR spectroscopy of disks can reveal double peaked line profiles characteristic of incomplete disks. This is described in detail in the GSMT science case5. ALMA will be able to image disk gaps at large radii in material that is too cold to be detected at near- and mid-IR wavelengths. A simulation from the ALMA science case is shown in Figure 9.2. Together, ALMA and the GMT will allow searches for young protoplanets over a wide range of ages, separations and contrast ratios. The southern hemisphere offers an ideal location for such studies as many of the richest star forming complexes pass overhead in Chile.

Figure 9.2 A protoplanetary disk with a tidal gap created by a Jupiter-like planet at 7AU from its central star. The simulated ALMA 350GHz image (right) clearly reveals the presence of the gap in the disk. From the ALMA science case.

9.3.3 Star Formation

Star formation is a key part of galaxy formation and is intimately connected to the process of planet formation. The physics of star formation remains poorly understood. The relevant region of the spectrum for physical studies of protostars and molecular clouds stretches from cm wavelengths in the radio to the near- and mid-IR. The mid-IR through mm regions of the spectrum are particularly critical as they sample a wide range of molecular species and are sensitive to dust emission at temperatures from 10 to 100K. ALMA will provide a vast new area of empirical studies of star formation within the

5 http://www.auranio.noao.edu/book/ch2/2_4.html GMT Science Case | Scientific Synergies with Other Major Facilities 111

Milky Way, the Magellanic Clouds, and other nearby galaxies. The GMT can play an important role in complementing mm and sub-mm studies with ALMA. The mid-IR high-resolution spectrograph on the GMT will allow access to higher energy transitions from a variety of molecular species. The GMT mid-IR imager, working in the diffraction limited laser AO mode, will provide imaging and resolutions comparable to those of ALMA and 2 - 3 times better than what can be achieved with present instruments.

9.4 GMT and the Low Frequency Radio Arrays

The Square Kilometer Array aims to explore the neutral gas content of the Universe through HI 21cm studies over the full range of look-back times. The core science driver is the epoch of reionization, but other science goals encompass a broad swath of contemporary astrophysics. Studies of star and planet formation, fundamental tests of general relativity and probes of cosmic magnetism are only part of the science goals of the SKA project. The development timescale for the SKA now appears to be beyond the horizon spanned by this document. There are, however, a number of SKA pathfinder experiments under development around the world and these address many or all of the same goals.

Developing a deep understanding of the reionization process and the time history of the epoch of reionization will require a multi-wavelength approach. Low-frequency arrays can detect signals in the redshifted HI background, but detection of Lyα absorption and emission signatures are essential for verification of the nature of the HI signal. ELTs will be particularly powerful for this application if the epoch of reionization is at z < 18.

One key science driver for low frequency arrays is the need for a deeper understanding of the baryon cycle in galaxies. Multi-wavelength observations that sample all phases of the gas and baryons locked-up in stars are essential. The GMT provides an ideal tool for tracking the ionized component of interstellar gas in galaxies, as well as outflowing winds, while SKA and SKA pathfinders will trace neutral gas within galaxies, cold inflow and tidal stripping of disks and outer envelopes. Together, with ALMA, the GMT, low frequency arrays, and Chandra/XMM and next generation x-ray observatories, we will be able to trace the full “life cycle” of baryons as they pass through the various phases of the ISM/IGM and through massive stars and drop out of the cycle into stellar sinks—low mass stars or stellar remnants.

9.5 LSST

9.5.1 Overview

The scientific motivation for LSST is broad and diverse. LSST builds on the success of the Sloan Digital Sky Survey (SDSS) and adds a new dimension through the time- domain. The LSST project characterizes their mission in the broadest terms as “making a color movie of the sky”. Regular multicolor imaging of a large fraction of the sky visible from Chile will produce both a legacy of deep and accurately calibrated multi-color

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photometric images, and a time-series of both photometry and astrometry of objects ranging from near Earth asteroids to the most distant quasars.

As a photometric survey facility, LSST will seed many scientific investigations, and a number of these will require ancillary data, particularly in the form of spectroscopy, to achieve their scientific aims. The depth of the LSST image catalogs is such that much of the new territory will be beyond the spectroscopic limits of current facilities. A cost- effective follow-up strategy will combine ELTs in Chile and Hawaii with multi object and time-series capabilities on smaller apertures distributed around the globe.

Table 9.2 LSST science goals and their connection to GMT

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On the left hand side of Table 9.2 we list the high-level science areas from the LSST science book along with the subtopics from each chapter. On the right we highlight areas where GMT can make significant contributions. The degree of synergy between GMT and LSST, as judged qualitatively by the importance of GMT observations to realizing the science goals on the right, is indicated by a green bar: The stronger the synergy the longer the bar.

There are a number of areas in which spectroscopy or AO imaging are essential to the LSST science goals and the synergy between the two facilities is high. There are other science areas, however, in which a relatively narrow-field large aperture telescope has limited utility. These are primarily questions for which very-large-area survey databases are used to derive large-scale or global quantities, such as the galaxy power spectrum. Even in these large-scale cosmological problems, however, the GMT will be helpful in determining the 3-D spatial distribution of galaxies through calibration of the LSST photometric redshifts via spectroscopy of faint galaxies with GMACS, NIRMOS, MANIFEST or similar survey spectrographs. Below we briefly consider a few of the science areas from the LSST science book where GMT and LSST can work together to advance common science goals.

9.5.2 Stellar Populations

With its large-area survey of the southern Milky Way, Magellanic Clouds and nearby galaxies, LSST will provide a vast photometric database of stellar populations in a range of environments. A deep understanding of their photometric and evolutionary properties is contingent on good abundance determinations. High- and intermediate-resolution spectroscopy with large apertures, and the GMT in particular, can provide this abundance information.

One of the science drivers for GMT involves understanding the early formation of the Milky Way and its halo through studies of extremely metal poor stars. This is also an important goal for LSST, and its photometric database can provide a fertile hunting ground for the most pristine stars in the Milky Way. This science is discussed in detail in Chapter 4 of this document.

The white-dwarf cooling sequences in galactic open clusters provide a chronometer for age dating the formation of the galactic disk. In Figure 9.3 (Figure 6.7 from the LSST science book) one can see the top of the white dwarf sequence in a number of southern Milky Way clusters. By probing ~2 magnitudes deeper, LSST will reach the bottom of the white dwarf cooling sequence in many of these and similar clusters. The GMT can provide spectroscopy of these faint stars to determine their masses and thus probe the initial-final stellar mass distribution in the early Milky Way. Targeted deep multi-color imaging with the GMT can also probe the bottom of the cooling sequence for the more distant clusters.

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Figure 9.3 Color-magnitude (CM) diagrams of six rich open star clusters observed as a part of the Canada-France-Hawaii Telescope Open Star Cluster Survey (Kalirai et al. 2001a). The clusters are arranged from oldest in the top-left corner (8Gyr) to the youngest in the bottom-right corner (100Myr). Each CM diagram presents a rich, long main sequence stretching from low mass stars with M > 0.5 MSun up through the turn-off, including post-main sequence evolutionary phases. The faint blue parts of each diagram illustrate a rich white dwarf cooling sequence (candidates shown with larger points). From the LSST Science Book.

9.5.3 Transients

One of the prime drivers for LSST and fast-developing frontiers of astrophysics is the time domain. LSST will not only provide far larger data sets for known classes of variable and transient phenomena, it will also probe regions of parameter space that are presently poorly explored. The time-domain survey teams will learn a great deal from photometric data alone, but much of the key astrophysics will require spectroscopy. Some of the transient targets will be faint, others will have short rise and decay times; some will be both faint and fast. A large aperture telescope operating in the same latitude and longitude with the ability to respond on short time-scales would provide an ideal synergy for this critical aspect of the LSST mission. The GMT with its single-object and survey class spectrographs can provide this capability.

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We consider two perspectives on this problem, following the approach in the LSST science book. The luminosity-decay time plane shown in Figure 9.4 shows the two well studied populations of explosive transient events—typical SNe and classical novae. The Palomar Transient Factory is currently exploring the ~8 magnitudes of parameter space in the gap. A number of anomalous SNe have been discovered in this region as well as other less well understood optical transients. LSST will cover this region of the luminosity timescale diagram with an unprecedented combination of area, depth and sampling. Spectroscopy of the various transients will be essential to understanding the physics involved in the various phenomena. The response time for these events is not terribly challenging, but careful coordination and prompt notification are essential to an effective follow-up strategy involving a range of apertures and spectral coverage.

Figure 9.4 The luminosity-decay time plane for cosmic transients on time-scales of 1 day or more. The dominant populations are classical novae with low luminosities and time scales of a few to a few tens of days and the typical supernovae with high luminosities and decay times ranging from a week to a month. The region between these two classes of transients is poorly explored. The Palomar Transient Factory and other time-domain projects are currently exploring this “gap” and LSST will survey it in detail. On the left we show the apparent magnitudes for a distance of 100Mpc. GMT should be able to cover the full gap to the distance of the Coma Cluster. Adapted from the LSST Science Book as derived from Kulkarni et al. (2007).

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A second and much less explored region of parameter space involves short timescale transient events. Short transients with high apparent luminosities (e.g. GRBs, XRBs) are known to us primarily through space-based large solid-angle monitoring experiments working at high energies. Low-energy and low-luminosity rapid transients are difficult to find. LSST, PTF and other time-domain experiments will explore new regions of parameter space (Figure 9.5). As was the case for GRBs, rapid spectroscopic follow-up from the ground is essential for understanding the nature of such objects. The way in which instruments are deployed on the GMT will allow rapid switching to one or more spectrographs in response to time-critical observations, such as rapid transients. The close proximity of GMT to LSST would allow close coordination for such observations.

Figure 9.5 The luminosity-decay time plane extended to very short timescales. GRB afterglows have very short decay constants and high luminosities. The region of parameter space consisting of short decay times and low luminosities remains unexplored. LSST coupled with rapid follow-up spectroscopy on GMT can explore less energetic processes with short dynamical times.

9.5.4 Galaxies and the Early Universe

The LSST will provide a large-area photometric database for studies of galaxy evolution. Much as the SDSS lead to a deeper understanding of the red and blue galaxy populations, LSST should provide a vital resource for galaxy evolution studies at intermediate and high redshifts. Spectroscopic redshifts and spectral diagnostics (e.g. star formation rate, ages, stellar populations, and velocities) will be critical to make use of this database, just as they have been with the Sloan survey data. LSST’s imaging survey will outstrip the spectroscopic capability of today’s 8m apertures. This is illustrated in Figure 9.6, adapted from the LSST Science Book, comparing the solid angle and depth of various imaging

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surveys. Recent ambitious surveys such as the NOAO Deep-Wide and COSMOS are pushing the limits of current spectrographs. Very deep spectroscopic surveys (e.g. GDDS, GMASS) with exposure times of ~30 - 50 hours reach AB magnitudes of ~26 with reasonable signal-to-noise ratios. The GMT will extend this by 1.5 - 2 magnitudes and thus will be well matched to the depth of the LSST survey database. Spectroscopy with GMACS, NIRMOS and other survey spectrographs will allow us to probe low-mass systems, rare objects, and to build large well characterized samples of galaxies over a wide range of environments.

Figure 9.6 Solid angle and 5 depth in AB magnitude for a number of extragalactic imaging surveys. The spectroscopic limit in the I-band for ~20h exposures for 4m, 8 - 10m telescopes, and the GMT are shown as dashed vertical lines. The GMT will be to reach the 10 limiting imaging depth of the LSST survey fields in practical exposure times.

High-redshift quasars provide us with unique probes of the early Universe. They are luminous signposts to high-density peaks in the underlying matter distribution and sites of early chemical enrichment. The space density of massive quasars at z > 6 imposes a strong constraint on models of structure and galaxy formation in the standard ΛCMD paradigm. At present, most of what we know of the z > 6 quasar population comes from the Sloan survey and some large area photographic surveys. LSST and large-area near-IR surveys (see below) will greatly expand our census of quasars at z > 5.

LSST will use color-color plots, or more accurately a multi-color space, to identify outliers from the stellar main- and giant-sequences (Figure 9.7). This has proven to be a very effective method of identifying quasars at high redshift. Other search techniques hold the hope of identifying quasars that lie within the stellar sequences in color-space.

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These include long-term variability and proper motion studies. Astrometric and variability studies suggest that a very large fraction of stellar objects that vary and do not move are distant quasars.

Large samples of distant quasars will not only improve our understanding of the evolution of quasar luminosity functions, the black-hole mass function, and high density peaks, they will also provide laboratories for studies of AGN physics and luminous probes of the intergalactic medium. The utility of distant quasars as probes of reionization is detailed in Chapter 7.

Figure 9.7 Two-color projections of a color-space used to separate quasars from stars and galaxies. Quasars at different redshifts occupy distinct regions of color space as the quasar spectral energy distribution is redshifted through the filters.

9.6 Southern Sky Surveys

A number of ambitious surveys of the southern sky are under development. The Dark Energy Survey (DES) will image 5000 square degrees in 5 colors on the CTIO 4m telescope. DES will be a forerunner to LSST in many respects and will produce a rich database of distant clusters, SNe, and galaxies across a broad range of redshifts. Among the fields that they intend to target are the SPT (South Pole Telescope) fields to identify massive galaxies clusters at intermediate redshifts. This will provide a valuable source of strong-lensing events that will require spectroscopy of large aperture telescopes to refine the mass models.

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The Sky Mapper project will provide a 2π steradian survey of the southern sky to identify large samples of supernovae at relatively low redshifts. The Sky Mapper survey will also provide a legacy sample of galaxy photometry to ~21 - 22 AB magnitudes per epoch. The survey cadence is spaced from a few hours to days, weeks, and 1 - 2 years. VISTA will provide a southern hemisphere sky survey in the J, H, and K-bands along with deep surveys in selected fields. As with other imaging surveys, spectroscopy of rare objects (e.g. z > 7 quasars) will be important to realizing the full potential of the survey.

9.7 Synergy with Space Based Missions

9.7.1 James Webb Space Telescope

In the spring of 2010, ESO held a four-day conference devoted to scientific synergy between JSWT and ELTs.6 The science case for JWST, like that for GMT, is built around two strong pillars: understanding the formation and early evolution of galaxies, and probing the formation and evolution of planetary systems and the stars they orbit. The strength of JWST is its great sensitivity at thermal wavelengths. While its angular resolution in the near-IR will be comparable to that of Hubble in the visible, it will not fall far short that of the ELTs operating at the diffraction limit.

The core JWST mission of probing first light and reionization is particularly well suited to synergistic work with larger apertures on the ground. NIRSPEC with its R = 100 mode will have unprecedented sensitivity to Lyα emission at high redshifts. Large targeted or blind surveys will identify hundreds of candidate Lyα emitters at z > 7. The GMT and other ELTs will have greater raw sensitivity to Lyα at R ~ 3 - 5000 for redshifts that do not place the emission lines under telluric water vapor lines or on top of strong OH emission lines. In Chapter 7 we discussed the power of GMT to explore Lyα at high redshifts. Webb can provide the key input survey datasets for GMT, presumably in legacy fields such as the CDFS, XMM-Subaru deep field, and other multi-wavelength survey fields.

In the field of exoplanet and protoplanetary and debris disk research JWST will benefit from its great sensitivity in the 2 – 10 micron regions. The low background offered by the cryogenic telescope above the terrestrial airglow will provide a sensitivity that cannot be matched on the ground. Webb will surely produce high sensitivity spectra and spectral energy distributions of planets and disks. GMT and other ELTs, however, will have better angular resolution and thus the smaller inner working angles critical for studies of massive close-in planets (e.g. roasters) and gas giants in the nearby star forming regions.

The contrast in angular resolution is illustrated in the image of the Beta Pictoris disk shown in Figure 9.8 below. This image, obtained with Gemini shows the β Pic debris disk and the PSFs for JWST and GMT at the same wavelengths for comparison.

6 Workshop presentations can be found at: http://www.eso.org/sci/meetings/jwstelt2010/program.html. 120 Scientific Synergies with Other Major Facilities | GMT Science Case

Figure 9.8 An 11 micron image of the β Pic debris disk taken with Gemini. The beam sizes for JWST and GMT at 11 microns are shown along with a bar 10AU in length. JWST will have unmatched sensitivity at mid-IR wavelengths, but GMT will have the angular resolution needed to probe the terrestrial planet zone.

The launch date of JWST is now fairly uncertain with dates as late as 2018 being discussed. The prospects for significant overlap between the lifetime of Webb and the GMT now look fairly good.

9.7.2 Euclid, WFIRST and Other Space-based Infrared Missions

A number of missions aimed at probing dark energy are under consideration by NASA, ESA and other space agencies. Euclid is a candidate ESA mission that would carry out a large spectroscopic survey of galaxies for baryon acoustic oscillation measurements of the expansion history. The 1.2m Euclid telescope would produce a legacy database for studies of galaxy evolution, distant quasars, clustering, and large-scale structure.

The Astro2010 New Worlds, New Horizons report recommended a large space-based mission aimed at dark energy and exoplanet science. WFIRST, a wide-field IR explorer, will carry out large surveys for SNe and faint galaxies for Baryon Acoustic Oscillation (BAO) studies. In the process, it will produce large multicolor catalogs of high galactic latitude sky. These will provide valuable survey databases for follow-up with ground- based ETLs and future space missions. WFIRST will operate in the 0.6 - 2.0 micron region of the spectrum. The legacy surveys carried out by Euclid and WFIRST should detect large numbers of z > 7 quasars, as tabulated below. Euclid was selected for development by ESA in October of 2011; launch is planned for 2019.

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Table 9.3 High redshift quasars expected from near-IR surveys

z > 7 z > 10 Survey Area (deg2) Depth (5-sigma, AB) QSO’s QSO’s UKIDSS-LAS 4000 Ks = 20.3 8 - VISTA-VHS 20,000 H = 20.6 40 - VISTA-VIKING 1500 H = 21.5 11 - VISTA-VIDEO 12 H = 24.0 1 - Euclid, wide (5 yr.) 15,000 H = 24.0 1406 23 WFIRST, deep (1 yr.) 2700 F3 = 25.9 904 17 WFIRST, wide (1 yr.) (4730) F3 = 25.3 - 25.5 1148 21

9.7.3 High-Energy Space Missions

The status of space missions targeted at high-energy astrophysical problems is uncertain. The Astro2010 panel did not rank the international X-ray observatory (IXO) as its top priority for space in the coming decade and so it is unlikely to get a new start soon.

Similarly, the Laser Interferometer Space Array (LISA) was not ranked and NASA has disbanded the LISA working group. ESA is currently considering concepts for scaled down versions for both missions. Similarly, a number of concepts are under development for a next generation gamma ray observatory. Having this capability in orbit during the first decade of GMT operations will be vital to some of the early Universe science discussed in Chapter 7.

9.7.4 Space-Based Exoplanet Missions

The Kepler mission is in the process of transforming the field of exoplanet research. Large numbers of planetary systems have been discovered and a large number of Earth- mass and super-Earth planets have been identified (e.g. Borucki et al. 2010). Ground- based follow-up of Kepler planets has been slow and this has been a limiting factor in removing contaminants (e.g. binary stars) from the transit database.

Kepler has observed a single deep field in Cygnus, thus it is not well place for follow-up observations from Chile. A number of targeted exoplanet missions are under consideration, but there are currently no approved NASA flights. The Transiting Exoplanet Survey Satellite (TESS) intends to survey ~2.5 million bright stars distributed over the entire sky in a two year program. They anticipate finding 1000 transiting exoplanets with masses ranging from super-Jupiters to Earth-mass rocky planets. NASA selected this mission for phase-A development as part of the 2011 call for explorer missions.

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Figure 9.9 Schematic of the coverage of the sky by PLATO in equatorial coordinates. Red areas indicate long pointings, blue areas show the step-and-stare phase covering a larger part of the sky. For comparison, the positions of the CoRoT and Kepler fields are shown. (From the PLATO “Red Book”)

Europe’s follow-on to Kepler - PLATO (“PLAnetary Transits and Oscillations of stars”) was not selected by the Science Programme Committee in the recent competition with Euclid and Solar Orbiter. PLATO’s core science goal was to discovery Earth-like planets—rocky bodies with radii near 1REarth and periods near one year. As an ESA mission, the survey fields were selected to ensure that some of them are well placed for follow-up observations from Chile. The fate of PLATO is unclear at this time; the ESA Science Programme Committee has decided to maintain PLATO as a possible contender for a future flight opportunity.

Both JWST and WFIRST, discussed above in the context of extragalactic science and star formation, will have important capabilities related to exoplanets. (Some of the important priorities for JWST in this field have been discussed above.) One of the key motivations for WFIRST is to use it to characterize Earth-mass planets via gravitational microlensing (Figure 9.10). The WFIRST mission should detect hundreds of low mass planets via their high amplitude but short lensing signatures on top of high magnification stellar microlensing events. The primary microlensing survey fields for WFIRST are in the galactic bulge and are thus well suited for follow-up or simultaneous observations from Chile.

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Figure 9.10 Gravitational microlensing by planets during a high magnification stellar microlensing event. The planet provides an additional sharp caustic when passing in front of the source resulting in short high magnification events superposed the longer stellar microlensing light curve. The mass of the planet can be deduced from the amplitude of the magnification event. From Queloz (2006).

9.8 Summary

We have provided an overview of scientific synergies between the GMT and a broad range of current and future facilities operating on the ground or from space. There are strong scientific connections among all of the facilities, but there are particularly strong links to large apertures that can provide spectroscopy in the visible and near-IR. This has been true for NASA’s great observatories and sky surveys (e.g. POSS, SDSS) and will very likely be the case for the next generation of survey missions, be they on the ground or operating above the atmosphere.

The maturing of adaptive optics brings and additional dimension to the utility of large telescopes for follow-up to survey and discovery missions. As AO becomes a more commonplace tool in the typical astronomer’s repertoire it will also become an integral part of follow-up to space-based missions and ground-based facilities alike. The ability to obtain images with angular resolutions of ~10mas will allow one to separate energy sources in composite systems (e.g. AGN and host galaxies, exoplanets) and derive characteristic sizes and positions with a new level of precision.

While we have attempted to be reasonably comprehensive, there will no doubt be many interactions between GMT and other facilities that we have not anticipated here.

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10 GMT Science Case Summary

We have presented the science case for the Giant Magellan Telescope as it occurs to our partner community as of mid 2011. Our motivation is a mix of contemporary science goals, the potential for new discoveries enabled by improved angular resolution and sensitivity, and multi-wavelength and multi-discipline scientific synergies with current and planned facilities.

The science case for the GMT does not differ radically from those for other ELTs and has strong overlap with the science motivation for JWST, ALMA and other general-purpose observatories. GMT does, however, have some unique strengths among the ELTs: Its fast focal ratio, somewhat smaller aperture, and Gregorian optical prescription lend themselves to large fields of view in seeing limited applications. The large primary mirror segments present both challenges and opportunities. The geometry of the primary and secondary mirror will allow for a very clean mid-IR telescope and enable some innovative approaches to the suppression of diffraction when trying to achieve very high contrast ratios at small inner working angles.

Throughout this document we have tried to make quantitative estimates of how GMT will address various current science questions and have, in many cases, simulated data to be obtained with the GMT and instruments currently under consideration. These have allowed us to sharpen our science focus and have fed-back into the instrument designs.

The GMT science case as presented in this book flows down to a series of requirements and definition-documents that drive the design of the facility. Naturally, this is an iterative process; neither the science case nor the requirements arose entirely independently, nor is one entirely a direct flow-down from the other. There is little doubt that the science motivation for the GMT, and ELTs in general, will evolve over time. It is also likely that as the GMT project progresses, technical considerations will lead to some changes in requirements. We hope that the science carried out in the first decade will be a mix of a subset of the investigations that we have outlined here and new and unanticipated lines of enquiry generated by new capabilities on the ground, in space, and in the minds of active scientists.

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11 Abbreviations and Acronyms

Below, listed in alphabetical order, is a list of common scientific abbreviations (or terms), acronyms and descriptions of past, present or proposed astronomical instruments or facilities found throughout the GMT Science Case

ΛCDM – Cold dark matter model with a cosmological ACS – Advanced Camera for Surveys AGB – Asymptotic Giant Branch AGN – Active Galactic Nuclei ALMA – Atacama Large Millimeter Array AO – Adaptive Optics AU – Astronomical Unit ASKAP – Australian Square Kilometer Array Pathfinder BAO – Baryon Acoustic Oscillation CDFS – Chandra Deep Field-South survey CDM – Cold Dark Matter CGM – Circumgalactic Medium Chandra/XMM – X-ray Multi-Mirror CMB – Cosmic Microwave Background CM diagram – Color Magnitude Diagram COBE – Cosmic Microwave Background Explorer COSMOS – Cosmological Evolution Survey CTIO – Cerro Tololo Interamerican Observatory DES – Dark Energy Survey DLA – Damped Lyman Alpha Absorber ELT – Extremely Large Telescope eROSITA – Extended Roentgen Survey with an Imaging Telescope Array Euclid – Proposed European near-IR sky survey satellite EXIST – Energetic X-ray Imaging Survey Telescope FIRE – Folded-port InfraRed Echellette (Magellan Telescope spectrometer) FOCAS – Faint Object Camera and Spectrograph on the Subaru Telescope FOV – Field of view FWHM – Full With at Half-Max Gaia – Proposed European astrometry satellite GDDS – Gemini Deep Deep Survey Gemini/FLAMINGOS-2 GLAO – Ground Layer Adaptive Optics GMASS – Galaxy Mass Assembly ultra-deep Spectroscopic Survey GRB – Gamma Ray Burst GSMT – Giant Segmented Mirror Telescope HARPS – High Accuracy Radial Velocity Planet Searcher HAT – Hungarian-made Automated Telescope HERMES – Herschel Multi-tiered Extragalactic Survey

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Herschel SPIRE – Spectral and Photometric Imaging Receiver, on the Herschel space observatory HST – Hubble Space Telescope IFU – Integrated Field Unit IMF – Initial Mass Function IGM – Intergalactic Medium ISM – Interstellar Medium JANUS – Joint Astrophysics Nascent Universe Satellite JWST – James Webb Space Telescope KBO – Kuiper Belt Object Keck/DEIMOS – Deep Imaging Multi-Object Spectrograph Keck/ESI – Echellette Spectrograph and Imager Keck/LRIS – Low Resolution Imaging Spectrometer Keck/MOSFIRE – Multi-Object Spectrometer for Infra-Red Exploration LAE – Lyman Alpha Emission LAMOST – Large Sky Area Multi-Object Fiber Spectroscopic Telescope LBG – Lyman Break Galaxies LBT – Large Binocular Telescope LCO – Las Campanas Observatory LISA – Laser Interferometer Space Array LSST – Large Synoptic Survey Telescope LTAO – Laser Tomography Adaptive Optics MDF – Metallicity Distribution Function MMT – Multiple Mirror Telescope MW – Milky Way MWA – Murchison Widefield Array NASA – National Aeronautics and Space Administration NFW – Navarro, Frenk and White NICMOS – Near Infrared Camera and Multi-Object Spectrometer (on HST) NOAO – National Optical Astronomical Observatory OSIRIS – Ohio State Infrared Imager/Spectrometer Pan-STARRS – Panoramic Survey Telescope and Rapid Response System PAPER – Precision Array to Probe the Epoch of Re-ionization PLATO – Planetary Transits and Oscillations of stars POSS – Palomar Observatory Sky Survey QSO – Quasi-stellar Object (aka Quasar) RGB – Red Giant Branch RSVP – ROTSE Supernova Verification Project ROTSE – Robot Optical Transient Search Experiment SDSS – Sloan Digital Sky Survey SDSS-III /BOSS – Baryon Oscillation Spectroscopic Survey SDSS-III/APOGEE – Apache Point Observatory Galactic Evolution Experiment SDSS-III/SEGUE-2 – Sloan Extension for Galactic Understanding and Exploration SDSS-III/MARVELS – Multi-object APO Radial Velocity Exoplanet Large-area Survey SINFONI – Spectrograph for INtegral Field Observations in the Near Infrared SKA – Square Kilometer Array

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SkyMapper – 1.3m telescope at Siding Spring Observatory, Australia SLSN – Super Luminous Supernova SLSNe – Super Luminous Supernovae SN – Supernova SNe – Supernovae SOAR – Southern Astrophysical Research Telescope Spaxel – Spectral pixel Spitzer/MIPS – Spitzer Space Telescope Multiband Imaging Photometer SPT – South Pole Telescope SVOM – Space Variable Objects Monitor Swift – Gamma ray burst satellite TESS – Transiting Exoplanet Survey Satellite TSS – Texas Supernova Search UDF – Hubble Ultra Deep Field UKIDSS – The UKIRT (United Kingdom Infrared Telescope) Infrared Deep Sky Survey VISTA – The Visible and Infrared Survey Telescope for Astronomy VLT – Very Large Telescope array VLT/KMOS – VLT K-Band Multi-Object Spectrograph VST – The VLT Survey Telescope WFC3 – Wide Field Camera 3 (on HST) WFIRST – Wide-Field Infrared Survey Telescope WMAP – Wilkinson Microwave Anisotropy Probe XRB – X-ray Burst Z – Metallicity z – Redshift

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