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ANNUAL in the First Billion REVIEWS Further Click here to view this article's online features: After the • Download figures as PPT slides • Navigate linked references • Download citations • Explore related articles • Search keywords Daniel P. Stark

Steward Observatory, University of Arizona, Tucson, Arizona 85721; email: [email protected]

Annu. Rev. Astron. Astrophys. 2016. 54:761–803 Keywords The Annual Review of and is , formation, galaxy , online at astro.annualreviews.org

This article’s doi: Abstract 10.1146/annurev-astro-081915-023417 In the past five years, deep imaging campaigns conducted with the Hubble Copyright c 2016 by Annual Reviews. (HST) and ground-based observatories have delivered large All rights reserved samples of galaxies at 6.5 < z < 10, providing our first glimpse of the census of formation activity in what is thought to be the heart of the reionization era. The space of luminous galaxies has been shown to decrease by 15–20× over 4 < z < 8. Over this same redshift interval, the faint-end slope of the UV function becomes steeper (α −2.0at z 7−8), revealing a dominant population of low-luminosity galaxies. Anal- ysis of multiwavelength imaging from HST and the demonstrates that z > 6 UV-selected galaxies are relatively compact with blue UV continuum slopes, low stellar , and large specific star for- mation rates. In the last , ALMA (the Atacama Large Millimeter Array) and ground-based spectrographs have begun to complement this picture, revealing minimal dust obscuration and hard radiation fields, and providing evidence for metal-poor ionized . Weak low- ab- sorption lines suggest a patchy distribution of neutral gas surrounds O and

Access provided by California Institute of Technology on 01/11/17. For personal use only. B , possibly aiding in the escape of ionizing radiation. Gamma ray burst Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org afterglows and Lyman-α surveys have provided evidence that the intergalac-

tic medium (IGM) evolves from mostly ionized at z 6−6.5(xHI < 0.2) to considerably neutral at z 7−8(xHI 0.3−0.8). The reionization history that emerges from considering the UV output of galaxies over 6 < z < 10 is consistent with these constraints on the IGM ionization state. The latest measurements suggest that galaxies can complete reionization by z 6and reproduce the optical depth faced by cosmic background if the luminosity function extends 4 mag below cur-

rent surveys and a moderate fraction ( fesc 0.2) of ionizing radiation escapes from galaxies.

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Contents 1. INTRODUCTION ...... 762 2.IDENTIFYINGEARLYSTAR-FORMINGGALAXIES...... 764 2.1.LymanBreakSelection...... 764 2.2. Lyman-α Emitters...... 766 2.3.DustyStar-FormingGalaxies...... 768 2.4.GammaRayBursts...... 768 3.THECENSUSOFGALAXIESINTHEFIRSTBILLIONYEARS...... 769 3.1. The Luminosity Function at 6 < z < 8...... 769 3.2. The Census of z 9–10 UV-Selected Dropouts ...... 774 3.3. The Lyman-α LuminosityFunction...... 774 3.4. The Contribution of Dusty Star-Forming Galaxies ...... 776 4.EARLYSTAR-FORMINGGALAXYPROPERTIES...... 777 4.1. UV Continuum Slopes ...... 777 4.2.StellarMassesandSpecificStarFormationRates...... 780 4.3. Insight into Massive Stellar Populations, Neutral Gas Covering Fractions, and Lyman-α VelocityOffsetsfromRest-FrameUVSpectroscopy...... 782 4.4. Conditions and Dust Properties from the Rest-Frame Submillimeter ...... 784 4.5. and Chemical Abundance Patterns ...... 786 4.6. Galaxy Sizes and Morphologies ...... 787 5. LYMAN-α EMITTERS AND GAMMA RAY BURSTS AS PROBES OFREIONIZATION...... 787 5.1. Evolution of Narrowband-Selected Lyman-α Emitting Galaxies over 5.7 < z < 8.8...... 787 5.2. The Lyman-α Emitter Fraction in z > 6LymanBreakGalaxies...... 790 5.3.GammaRayBurstAfterglows...... 792 6. EARLY GALAXY GROWTH AND CONTRIBUTION TOREIONIZATION...... 793 6.1. Cosmic Evolution of Rate Density and Stellar Density . . . 793 6.2. Contribution to Reionization ...... 794

1. INTRODUCTION Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org One of the longstanding goals of is to construct a coherent picture of cosmic history, tracking the from its origins to the day. The cosmic microwave background (CMB) provides our first snapshot, revealing the universe just after the epoch of re- combination roughly 400,000 years after the Big Bang. This marks the beginning of the cosmic dark ages with neutral filling most of the universe. Deep images from the (HST) provide our next picture nearly one billion years later, revealing a dramat- ically different landscape. Star-forming galaxies are abundant; some appear to have already built 10 up 10 M of stellar mass. The absorption lines in spectra highlight another fundamental change: the hydrogen that filled the universe after recombination has become highly ionized in the intergalactic medium (IGM) by a redshift z 6, one billion years after the Big Bang. The current observational frontier in the study of cosmic history aims to piece together these two radically different views of the universe, addressing when and how galaxies first formed and

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built up their stellar content, and determining whether the of the first galaxies pro- vided the ionizing radiation necessary to reionize intergalactic hydrogen. The existing theoretical framework suggests that the process begins following the emergence of the first generation of chemically pristine stars formed in minihalos at z 20−30. The formation of these so-called Pop- ulation III (Pop III) stars terminates the cosmic dark ages and initiates the gradual enrichment of the universe with metals. The of the first stars remains a of debate. Theoretical work initially suggested a top-heavy mass function (see Bromm & Yoshida 2011 for a review), but more recent simulations have indicated that the Pop III stars could be much lower in mass (e.g., Stacy & Bromm 2014). Dwarf galaxies eventually form in low-mass halos containing gas that has been polluted by earlier generations of stars, and are thus above the metallicity threshold necessary for forming normal low-mass Population II stars. Over the next billion years, galaxies are expected to rapidly accrete gas, growing considerably in mass. The Lyman continuum (LyC) radiation released from these early galaxies ionizes their intergalactic surroundings. Over , bubbles of ionized hydrogen grow and overlap around overdensities in the matter distribution, leading to the reionization of hydrogen. This marks the important point at which has impacted every in the universe. In the past five years, the cosmic frontier has been pushed back to a redshift z 10, revolu- tionizing our view of early galaxies and allowing the first direct tests of the theoretical picture described above. Much of this progress has been achieved thanks to deep imaging campaigns con- ducted with the Wide Field Camera 3 (WFC3) onboard HST following its installation in 2009. The unprecedented near-IR sensitivity of WFC3 has enabled more than a thousand galaxies to be identified within the first billion years, providing a census of star formation activity in the redshift range 6 < z < 10, and allowing investigation of their contribution to reionization. A variety of multiwavelength constraints targeting UV-selected galaxies, quasar metal absorption lines, (sub)millimeter selected galaxies, and gamma ray bursts (GRBs) have provided new insight into the physical nature of these early star-forming systems. Meanwhile, Lyman-α (Lyα) surveys and GRB afterglow spectra are constraining the timescale of reionization, complementing new constraints on the scattering optical depth faced by CMB photons ( Collaboration 2015, 2016) and a growing number of quasar absorption line spectra at z > 6.5 (e.g., see Becker et al. 2015 for a review). The rapid progress promises to accelerate in the coming years following the completion of a suite of ambitious new facilities, including the James Webb Space Telescope ( JWST), the Atacama Large Millimeter Array (ALMA), the Wide Field Infrared Survey Telescope (WFIRST), and the next generation of ground-based optical/infrared Extremely Large . With these facilities on the near horizon, it is a particularly valuable time to summarize the physical picture that has emerged following the past five years of observations. This manuscript builds on the Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org review of Dunlop (2013) and complements several recent theoretical reviews on reionization (e.g., Lidz 2016) and early galaxy formation (e.g., Bromm & Yoshida 2011, Milosavljevic&Safranek-´ Shrader 2016). The plan is as follows. In Section 2, we introduce the samples of galaxies that have been identified in the first billion years, reviewing the selection techniques and survey parameters. In Section 3, we describe the census of star-forming galaxies that has emerged from studies of UV-selected dropouts, Lyα emitters, and dust-enshrouded massive galaxies. We describe what is currently known about the physical nature of early galaxies in Section 4. We begin the discussion of reionization in Section 5, presenting emerging constraints on the IGM ionization state from Lyα surveys and deep GRB afterglow spectra, and in Section 6, we synthesize the results from earlier sections by presenting the global buildup of star formation and stellar mass and reviewing the latest constraints on the contribution of galaxies to reionization. We close with a short summary of key points and discussion of future issues.

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2. IDENTIFYING EARLY STAR-FORMING GALAXIES

2.1. Lyman Break Selection Much of the progress discussed in this review has been made possible by large samples of distant galaxies identified via Lyman break selection. The method is based on the imprint of hydrogen absorption on the rest-frame UV spectra of distant galaxies. In absence of significant quantities of dust, star-forming galaxies will have blue UV continuum spectra, powered by massive O and B stars. Neutral hydrogen within the galaxy and in the IGM absorbs at below

the Lyman limit, λrest = 912 A˚ , creating a sharp break that gives high redshift galaxies distinct rest-frame UV colors, allowing for efficient selection via multiwavelength imaging. The Lyman break galaxy (LBG) technique first achieved for z 3 galaxies (see Shapley 2011 for a review) using U, G, and R filters. The LBG selection is easily extended to earlier cosmic epochs by using a redder set of filters and accounting for the fact that the spectral discontinuity occurs

at the rest-frame of Lyα, λrest = 1216 A˚ , owing to the increased opacity provided by intergalactic hydrogen. At a redshift of z 6, just under one billion years after the Big Bang, the Lyα spectral break is shifted to λobs = 0.8 µm, between the i -andz-band filters. Galaxies at z 6arethusi -band dropouts, requiring imaging in the i-andz-band filters to identify the sharp Lyα break and longer-wavelength near-IR imaging to ensure the presence of moderately blue rest-frame UV continuum colors. The first substantial breakthrough into the first billion years came following the HST Servicing Mission 3B in March 2002, which installed the Advanced Camera for Surveys (ACS) and delivered a new cooling system for the Near Infrared Camera and Multi-Object (NICMOS). The Wide Field Channel of ACS provided a larger field of view (202 arcsec × 202 arcsec) and improved sensitivity with respect to WFPC2 (the Wide-Field Planetary Camera 2), resulting in an order-of-magnitude improvement in survey efficiency. It also gave HST the capability to image in the z band for the first time, opening the door for robust identification of z 6i -band dropouts. A series of deep multiwavelength imaging campaigns were initiated with ACS soon after its installation. The Great Observatories Origins Deep Survey (GOODS) program (Giavalisco et al. 2004) imaged the Chandra Deep Field South (CDF-S) and Hubble Deep Field North (HDF- 2 N) in four filters (B435,V606, i775,andz850), providing roughly 320 arcmin to 5σ sensitivity limits of mAB 27. Application of a simple i -band dropout selection to the first few epochs of GOODS imaging quickly led to discovery of the first 10 bright (z850 < 25.6) z 6 LBGs (Stanway et al. 2003, Dickinson et al. 2004). In 2004, ACS delivered the deepest ever optical images of the sky as

part of the Hubble Ultra Deep Field (HUDF) program, reaching mAB = 29 in the B435,V606, i775, 2 and z850-bands over a 11.3 arcmin region of the CDF-S field (Bunker et al. 2004, Beckwith et al. 2006, Bouwens et al. 2006). Throughout the following five years, attention continued to focus Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org on analysis of z 4−6 LBGs in the HUDF and GOODS fields. By 2007, more than 600 z 6 LBGs had been identified in multiwavelength HST imaging (e.g., Bouwens et al. 2007). Deep Spitzer Space Telescope Infrared Array Camera (IRAC) (Fazio et al. 2004) images of the GOODS fields emerged in 2005, enabling the first measurements of the assembled stellar mass in z 5−6 galaxies (e.g., Egami et al. 2005, Eyles et al. 2005, Stark et al. 2007a). The depth of the 3.6- and

4.5-µm IRAC imaging in the two GOODS fields (mAB 26.0) enabled detection of many of the brightest z > 4 galaxies discovered in the HST imaging. In spite of the great progress at z 6, the redshift frontier remained firmly situated at z 7

in 2008. Although deep NICMOS imaging enabled identification of a handful of z 7 z850-band dropouts (e.g., Bouwens et al. 2004b, 2008), the samples were too small for a reliable census and the detections were not of sufficient signal-to-noise (S/N) for detailed spectral distribution (SED) analysis. This immediately changed following the delivery of WFC3 on HST during the

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Servicing Mission 4 in May 2009. The larger field of view (4.8 arcmin2) and improved sensitivity of the infrared channel led to a 40× improvement in survey efficiency compared with NICMOS. Shortly after its installation, WFC3/IR was used to image the HUDF and associated parallel

fields in three filters (Y105,J125,H160) (Bouwens et al. 2011), delivering the deepest near-IR view of the universe, which in turn allowed identification of the first large samples of faint galaxies at z 7−8. In 2012, the HUDF was again observed by WFC3/IR (Koekemoer et al. 2013), providing

increased depth and a new filter ( JH140). Illingworth et al. (2013) combined all WFC3/IR and ACS imaging in the 4.7-arcmin2 area of overlap between the various HUDF surveys and renamed

this subregion the Extreme Deep Field (XDF). The HUDF images reach mAB = 29−30 in the near-IR filters, allowing galaxies with absolute magnitudes as faint as MUV =−17 to be identified at z 7. Moderate-depth WFC3/IR surveys over wider areas have provided equally important constraints on the census of early galaxies. The Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS; Grogin et al. 2011, Koekemoer et al. 2011) imaged subregions of five deep extragalactic survey fields (EGS, the Extended Groth Strip; , the Cosmological Evolution Survey; HDF-N; CDF-S; UDS, Ultra-Deep Survey). The survey is separated into Deep and Wide components. The CANDELS-Deep fields (133 arcmin2 within the HDF-N and CDF-

S) were imaged in Y105,J125,andH160 to limiting depths of mAB = 27.5−27.8. The CANDELS- 2 Wide fields cover a much larger area (553 arcmin )intwofilters(J125,H160), reaching limiting depths of mAB = 26.6−26.8. Additional wide area constraints come from the Early Release imaging of the CDF-S (Windhorst et al. 2011) and the 450-arcmin2 BoRG/HIPPIES (Brightest of Reionizing Galaxies/Hubble Infrared Pure Parallel Imaging Extragalactic Survey) HST parallel imaging fields (Trenti et al. 2011, Yan et al. 2011). Together the wide-area HST

surveys have delivered large samples of galaxies with absolute magnitudes in the range MUV =−21 −  to 19, helping to anchor the luminosity function near the characteristic UV luminosity, LUV. When all datasets described above are combined, HST photometric samples now include 300–500 galaxies at z 7 and 100–200 galaxies at z 8 (e.g., Bouwens et al. 2015b, Finkelstein et al. 2015). These photometric samples are expected to have contamination levels of 7% (z 7) and 10% (z 8). Accurate characterization of the UV luminosity function (LF) also requires robust constraints >  on the space density of L LUV galaxies. Although the wide-area HST surveys described above certainly help in this endeavor, the number counts drop off rapidly at the bright end, causing

uncertainty in the census of the most luminous (MUV =−23 to −21) galaxies. Because ground- based near-IR imaging campaigns can more efficiently survey very large areas (>1deg2)than HST (albeit at shallower depths), they provide a valuable means of locating rare luminous galaxies (McLure et al. 2009, Ouchi et al. 2009). The most recent ground-based constraints (Bowler et al. 2014, 2015) are based on deep imaging from the UltraVISTA (Ultra Deep Survey with Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org the VISTA telescope) and UKIDSS (United Kingdom Infrared Telescope) UDS campaigns. The

UltraVISTA survey provides Y-, J-, H-, and Ks -band imaging in the COSMOS field, reaching 2 2 mAB 25.8 over 0.62 deg with an additional 0.29 deg covered to a depth of mAB 25. The UDS field has been imaged to a depth of mAB 25 in J, H, and Ks , with Y-band imaging reaching 2 mAB 25.3 over a total 0.75 deg . Application of the Lyman break selection to these data has revealed 266 objects at 5.5 < z < 6.5 and a further 34 at 6.5 < z < 7.5 (Bowler et al. 2014, 2015), providing valuable constraints on the bright end of the luminosity function. The current observational frontier lies at z 9 − 11. At these , the Lyα forest strongly

attenuates light below 1.2 µm, leaving only one filter (H160) that is guaranteed to be uncontam- inated by IGM absorption. Without two independent filters to probe the UV continuum slope,

it is difficult to remove lower redshift (z 1−3) contaminants. The JH140 imaging in the HUDF helps selection of z 9−11 galaxies (e.g., Ellis et al. 2013, Oesch et al. 2013, McLure et al. 2013),

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but the JH140 depth is not adequate in most deep fields to be of use. This situation can alternatively be remedied for the small number of z 9 galaxies which are bright enough for Spitzer/IRAC to deliver a meaningful flux constraint in the 3.6- and 4.5-µm channels (hereafter [3.6] and [4.5]) (e.g., Oesch et al. 2014, 2016; Bouwens et al. 2015c). The magnification provided by strong grav- itational lensing can boost the flux of distant galaxies enough to make them visible with IRAC, making lensing particularly advantageous for current surveys of z 9−10 galaxies. The Cluster Lensing and survey with Hubble (CLASH; Postman et al. 2012) has delivered two

very bright (H160 = 25.7−25.9) galaxies with robust photometric redshifts that place them at z 9−11 (e.g., Zheng et al. 2012, Coe et al. 2013). Such bright objects provide a unique opportu- nity for detailed investigation. Currently gravitational lensing efforts are focused on the Hubble

Frontier Fields (HFF; Lotz et al. 2016) which is delivering ultradeep imaging (mAB 28.9) of six high-magnification cluster fields, with the primary goal of constraining the abundance of low luminosity galaxies at z 7−10. Because most known z > 6 galaxies have very faint continuum fluxes, spectroscopic confir- mation has, not surprisingly, proven difficult. At the highest redshifts (z > 7), the problem is compounded by the attenuation of Lyα by intergalactic hydrogen and the practical difficulties of observing in the near-IR from the ground. Nevertheless, large spectroscopic campaigns with Keck and the VLT (Very Large Telescope) have contributed greatly to our understanding of early galaxies and reionization (e.g., Vanzella et al. 2009; Stark et al. 2010, 2011; Cassata et al. 2015). Current spectroscopic samples consist of roughly 350 galaxies in the redshift range z 4−7. Over the past five years, a great deal of effort has been put into spectroscopic observations of Lyα in z 7−9 LBGs. Thus far, ten UV or Lyα-selected galaxies have been confirmed with redshifts 7 < z < 8 (Vanzella et al. 2011, Ono et al. 2012, Schenker et al. 2012, Shibuya et al. 2012, Oesch et al. 2015b, Roberts-Borsani et al. 2016, Song et al. 2016b, Stark et al. 2016) and one system has been confirmed at z > 8 (Zitrin et al. 2015). The latter object lies at z = 8.68 and is currently the highest-redshift spectroscopically confirmed galaxy. These surveys have demonstrated that the success rate for spectroscopic confirmation at z > 7 is much lower than at z 4−6, reflecting very low-equivalent-width Lyα emission from early galaxies. We come back to this point in Section 5 when discussing the use of Lyα as a probe of reionization. Although several low-z interlopers have been identified in surveys at z 4−6, the fraction of these galaxies appears small (<10%), similar to contamination levels estimated from simulations.

2.2. Lyman-α Emitters Nearly 50 years ago, the Lyα emission line was suggested as a promising means of identifying high-redshift galaxies (Partridge & Peebles 1967). Since the late 1990s, when the technique first

Access provided by California Institute of Technology on 01/11/17. For personal use only. > α Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org achieved success at z 5 (Dey et al. 1998), surveys of Ly emitting galaxies (LAEs) have played an important role in driving our understanding of early star formation and cosmic reionization. At z > 5, Lyα is shifted to red wavelengths (>7000 A˚ ) where a forest of bright OH sky lines are present. Because of the increased noise associated with the OH lines, conventional blank-field spectroscopic searches for LAEs at z > 5 are highly inefficient. The most successful approach instead involves targeting narrow spectral windows in which OH sky lines are mostly absent. The widest gaps in the OH forest span 100−200 A˚ and are situated at 8200, 9200, 9800, and 10050 A˚ , corresponding to Lyα emission at z = 5.7, 6.5, 7.0, and 7.3. By manufacturing narrowband filters to fill these dark regions, it is possible to obtain very deep images which can isolate strong Lyα emission from galaxies with relatively faint stellar continuum emission. Most surveys require LAE candidates to be 1 mag brighter in the narrowband bandpass than in the adjacent broadband filter, corresponding to a rest-frame Lyα equivalent width (EW) limit of 20 A˚ . Because the majority of line emitters in a narrowband image will be low-redshift interlopers ([OII], [OIII], or 766 Stark AA54CH18-Stark ARI 25 August 2016 20:52

Hα at z 0.5−1.5), additional color-selection criteria are frequently applied to isolate a robust photometric sample of LAEs. It is customary to require nondetection in blue optical broadband imaging, consistent with the strong Lyα continuum break expected for z > 5 galaxies. Foreground objects are also flagged and removed through the presence of emission lines in follow-up optical . By the end of the 1990s, Lyα selection successfully began to identify star-forming galaxies at z > 5 (e.g., Dey et al. 1998, Hu et al. 1998), culminating in the discovery of a Lyα emitter at z = 6.56 (Hu et al. 2002), the first spectroscopically confirmed star-forming galaxy beyond z = 6. Early searches for LAEs also utilized slit spectroscopy, which covers a wider redshift range at the expense of a smaller area than narrowband imaging surveys. A particularly innovative example involved searching for highly magnified gravitationally lensed LAEs by mapping the critical lines of massive galaxy clusters using long-slit spectrographs on Keck (Ellis et al. 2001, Santos et al. 2004, Stark et al. 2007b). As wide-field CCD imaging cameras became more common on large ground-based telescopes, the efficiency of the narrowband surveys increased significantly. One of the first LAE campaigns to take advantage of wide-format optical cameras was the Large Area Lyα survey, providing some of the first constraints on the evolution in the LAE LF over 4.5 < z < 6.5 (e.g., Rhoads & Malhotra 2001). The next major advance came with the installation of the 34×27 Subaru/Suprime-Cam mosaic CCD camera (Miyazaki et al. 2002) on the 8.2-m Subaru telescope, enabling substantial improvements in the areal coverage and depth of LAE surveys, leading to a flurry of activity. Initial Suprime-Cam campaigns obtained deep narrowband imaging in the NB816 and NB912 filters (corresponding to Lyα at z = 5.7 and 6.6) and multiwavelength broadband imaging in several deep fields. The Suprime-Cam surveys provided 0.2 deg2 of deep narrowband imaging in the Subaru Deep Field (SDF; Kashikawa et al. 2006, 2011; Shimasaku et al. 2006), 1 deg2 of imaging in the Subaru/XMM-Newton Deep Field Survey (SXDS) field (Ouchi et al. 2008, 2010), nearly 2 deg2 of imaging toward the COSMOS field (e.g., Taniguchi et al. 2007), and a combined 1.25 deg2 area across HDF-N, , SSA22, and SSA17 (Hu et al. 2010). Together these surveys have led to the discovery of more than 700 (300) LAE candidates at z = 5.7 (6.5) in comoving volumes in excess of several ×106 Mpc3. Spectroscopic follow-up of the Suprime-Cam samples has confirmed 211 LAEs at z = 5.7andafurther91atz = 6.5 (e.g., Ouchi et al. 2008, 2010; Hu et al. 2010; Shimasaku et al. 2006; Kashikawa et al. 2006, 2011; Mallery et al. 2012). Narrowband 5σ sensitivities typically probe Lyα down to 42 −1 LLyα 2.5 × 10 erg s . Narrowband imaging surveys were extended to z 7 in 2005 with the construction of a Suprime-Cam narrowband filter (NB973) centered at 9755 A˚ . The initial NB973 imaging of

the SDF reached down to mNB = 24.9 and led to the discovery of a LAE at z = 6.96 (Iye et al. 2006). This object remained the highest-redshift spectroscopically confirmed galaxy for 6 years,

Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org only being surpassed after HST began delivering robust UV-dropouts at z 7. A further six z 7 LAE candidates were reported in a survey of a 0.13-deg2 subregion of COSMOS using Magellan/IMACS (the Inamori-Magellan Areal Camera and Spectrograph) with a custom-built narrowband filter centered at 9680 A˚ (Hibon et al. 2011). One of these systems was later confirmed spectroscopically at z = 6.944 (Rhoads et al. 2012). The installation of fully depleted Hamamatsu CCDs on Suprime-Cam made it possible to extend Subaru narrowband surveys to 1 µm. A new narrowband filter (NB1006) manufactured with a central wavelength of 10052 A˚ enabled the first constraints on the volume density of LAEs out to z = 7.3. Four candidate LAEs were reported in Shibuya et al. (2012) following NB1006 imaging of the SDF and SXDF. Spectroscopic follow-up confirmed the redshift of one system at z = 7.215. Konno et al. (2014) obtained the deepest

NB1006 images to date, finding 7 LAE candidates down to mNB = 25.1 in COSMOS and the SXDS. The Konno et al. (2014) survey probes down to 2.4×1042 erg s−1 in Lyα luminosity, four deeper than previous LAE surveys at z 7.0−7.3. There have been several narrowband www.annualreviews.org • Galaxies in the First Billion Years 767 AA54CH18-Stark ARI 25 August 2016 20:52

surveys for LAEs at redshifts (z = 7.7, z = 8.8) where Lyα is redshifted to dark gaps in the near- IR OH sky background (e.g., Hibon et al. 2010, Clement´ et al. 2012). Although several promising candidates have been discovered, none have been spectroscopically confirmed. The narrowband surveys described above typically do not reach low enough luminosities to constrain the faint end slope of the Lyα LF. A complementary technique to find fainter LAEs has begun to yield dividends in recent years. The basic idea is to use a grism and custom blocking filter to target Lyα emitters in the 100–200 A˚ dark gaps in the OH sky background. The spectral resolution of the grism (10 A˚ ) reduces the sky background by roughly an (at the expense of a smaller survey area), allowing very low-luminosity LAEs to be identified. By using a highly multiplexed slitmask with a large field of view, it is possible to survey areas of up to 60 arcmin2 per pointing. This technique first achieved success (Martin et al. 2008) using the large field of view provided by Magellan/IMACS to target z = 5.7 LAEs. Spectroscopic follow-up has confirmed more than 20 candidates as LAEs and allowed characterization of the foreground contaminant population (Henry et al. 2012, Dressler et al. 2015). The Lyα luminosities probed by these multislit narrowband surveys are as low as 0.9×1042 erg s−1, well below the faintest LAEs in narrowband imaging surveys.

2.3. Dusty Star-Forming Galaxies Dust-enshrouded massive starbursts are commonly missed from galaxy selections in the rest- frame UV (see Casey et al. 2014 for a review). Such objects form stars at prodigious rates (up to −1 1,000 M yr ) and contribute significantly to the cosmic star formation rate density (SFRD) at z 1−2. In the past few years, the first samples of dusty, massive star-forming galaxies have been confirmed at z > 5 (Capak et al. 2011; Walter et al. 2012a; Riechers et al. 2013, 2014). The first dust-obscured starburst confirmed at z > 6 was reported by Riechers et al. (2013). The galaxy was identified in Herschel/SPIRE (Spectral and Photometric Imaging Receiver) imaging taken as part of the HerMES (Herschel Multi-tiered Extragalactic Survey) blank field survey. The short submillimeter wavelengths probed by SPIRE (250, 350, and 500 µm) sample the peak of the dust emission SED at very high redshift, making it possible to isolate very high-redshift galaxies with simple color selections. In this manner, Riechers et al. (2013) identified five z > 3.5 dusty galaxy candidates down to a flux limit of 30 mJy at 500 µm over an area of 21 deg2. The redshift of the most promising candidate, HFLS 3, was confirmed by Riechers et al. (2013) to be at z = 6.3369 through detection of a large number of molecular and atomic fine-structure cooling lines. Progress is also being made from surveys conducted with the South Pole Telescope (SPT). The most recent SPT catalog contains 39 spectroscopically confirmed dusty star-forming galaxies (most of which are gravitationally lensed), including five systems at 5 < z < 6 (Strandet et al. 2016). The galaxies

Access provided by California Institute of Technology on 01/11/17. For personal use only. 2 σ Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org were selected at 1.4 mm from the 2,500 deg SPT-SZ survey, which provides a 5 sensitivity of

S1.4mm 20 mJy. Synchrotron sources are removed from this initial selection using a simple flux ratio criterion (S1.4mm/S2.0mm > 2) (see Weiß et al. 2013 for detailed discussion of SPT selection), and blind spectral scans with ALMA are then use to confirm redshifts spectroscopically. The long-wavelength selection of SPT translates into a higher median redshift (z = 3.9 ± 0.4; Strandet et al. 2016) than is found in populations selected at shorter wavelengths, providing an efficient means of identifying the highest-redshift dusty star-forming galaxies.

2.4. Gamma Ray Bursts GRBs provide a valuable method of studying early galaxies and reionization. Owing to the extreme luminosities of their afterglows, GRBs can be detected out to z 20 with current facilities (Gou

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et al. 2004). The afterglows commonly reach flux levels that are up to a million times as bright as the z > 6 galaxies in the UDF, providing a rare opportunity to measure spectroscopic redshifts (independently of the presence of Lyα emission) and probe the surrounding gaseous environment of the host galaxy in absorption. Because the afterglow spectrum is intrinsically featureless, GRBs are ideal laboratories for studying the chemical enrichment, hydrogen column , and extinction laws of the ISM (e.g., Fynbo et al. 2009, Berger et al. 2014), and in some cases the ionization state of the IGM (e.g., Miralda-Escude´ 1998, McQuinn et al. 2008). One advantage of GRBs as probes of the IGM is that unlike , they do not modify their environments on large scales. Because GRBs are powered by individual massive stellar systems, they should probe the entirety of the UV luminosity function, including feeble galaxies that are too faint to be seen in the deep imaging surveys discussed in Section 2.1. The evolution in the GRB space density with redshift may thus be able to provide a useful measure of the cosmic star formation history, complementing inferences from conventional flux-limited surveys (e.g., Robertson & Ellis 2012). In addition, detailed study of GRB host galaxies provides one of our only windows on the formation of the low-mass galaxies that are thought to dominate the ionizing output in the early universe. As of 2016, there are five spectroscopically confirmed GRBs at z > 5.9. The first of these to be discovered was GRB05094, a burst at z = 6.295 with an afterglow that was as bright as J = 17.5 (e.g., Kawai et al. 2006, Totani et al. 2006). Several years later, GRB080913 was confirmed at z = 6.733 (Greiner et al. 2009). And in 2009, GRB090423 was identified at z 8.23 (Tanvir et al. 2009, Salvaterra et al. 2009). For six years, this GRB remained the highest-redshift spectroscopically confirmed object known. Most recently, the discovery of GRB130606A at z = 5.913 (Chornock et al. 2013, Totani et al. 2014) and GRB14051A at z 6.33 (Chornock et al. 2014) has provided the highest S/N absorption line spectra, opening the door for GRBs to deliver unique constraints on the IGM ionization state at z > 5.9. In addition to these spectroscopically confirmed systems, a potentially very high-redshift GRB was reported by Cucchiara et al. (2011) with a photometric redshift of z 9.4. In this review, we discuss how these GRBs are contributing to our understanding of the ISM and dust properties of early galaxies (Section 4.4), the IGM ionization state at z > 6 (Section 5.3), and the SFRD of the universe at z > 5 (Section 6.1).

3. THE CENSUS OF GALAXIES IN THE FIRST BILLION YEARS The galaxy samples described in Section 2 have enabled the first constraints on the abundance of star-forming systems in the first billion years. In this section, we describe the latest measurements of the luminosity functions of LBGs and LAEs at very high redshift, discuss current knowledge of the prevalence of dusty galaxies, and comment on implications for the assembly of galaxies in the early universe. Discussion of the global SFRD is deferred until Section 6, following our discussion Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org of the dust content of high-redshift galaxies in Section 4.

3.1. The Ultraviolet Luminosity Function at 6 < z < 8 The UV LF of UV continuum dropout galaxies currently provides our most complete census of star-forming galaxies at z > 6. The UV LF is typically parameterized by a Schechter function (Schechter 1976),     α dn φ L − /  = φ(L) = exp L L (1) dL L L where φ is the characteristic volume density, L is the characteristic luminosity, and α is the faint- end slope. Schechter functions are good descriptions for populations that follow a near power-law slope α at the faint end and exhibit an exponential cutoff above the characteristic luminosity L.

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For high-redshift galaxies, the Schechter function is frequently given in terms of the absolute magnitude rather than the luminosity,

ln 10  . − α+ . − φ(M) = φ (100 4(M M ))( 1) exp[−100 4(M M )], (2) 2.5 where M  is the characteristic absolute magnitude. The absolute magnitude used in the UV LF typically refers to the luminosity at a rest-frame wavelength of 1500 A˚ . Measurements of the UV luminosity function have steadily improved over the past ten years (e.g., Bunker et al. 2004; Beckwith et al. 2006; Bouwens et al. 2006, 2007, 2011; Finkelstein et al. 2010; McLure et al. 2009, 2010; Schenker et al. 2013b). The most recent z > 4 LF determinations derived from HST imaging (Bouwens et al. 2015b, Finkelstein et al. 2015) are based on 4,000– 6,000 z 4 galaxies, 2,000–3,000 z 5 galaxies, 700–900 z 6 galaxies, 300–500 z 7 galaxies, and 100–200 z 8 galaxies. The Bouwens et al. (2015b) study is the largest effort conducted to date, including galaxies in all five CANDELS fields, the BoRG/HIPPIES fields, and the HUDF/XDF and its associated parallels, allowing the UV LF to be characterized over a large dynamic range

(M UV 6atz 6). The HST samples are complemented by ground-based imaging surveys that place valuable constraints on the space density of galaxies as bright as M UV −23 (e.g., Bowler et al. 2014, 2015).

z~4 z~5 z~6 10–2 ) –3 10–3 Mpc –1 10–4

10–5 McClure et al. (2009) Bowler et al. (2015)

Number (mag 10–6 Bouwens et al. (2015b) Bouwens et al. (2015b) Bouwens et al. (2015b) Finkelstein et al. (2015) Finkelstein et al. (2015) Finkelstein et al. (2015)

z~7 z~8 z~10 –2 10 McLeod et al. (2016) )

–3 Oesch et al. (2013) 10–3 Bouwens et al. (2015b)

Mpc Bouwens et al. (2015c) –1 10–4

Ouchi et al. (2009) 10–5 Bowler et al. (2015)

Access provided by California Institute of Technology on 01/11/17. For personal use only. Bouwens et al. (2015b)

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org Bouwens et al. (2015b) Number (mag 10–6 Finkelstein et al. (2015) Finkelstein et al. (2015) McLure et al. (2013) McLure et al. (2013)

–23 –22 –21 –20 –19 –18 –17 –23 –22 –21 –20 –19 –18 –17 –23 –22 –21 –20 –19 –18 –17 M M M UV UV UV

Figure 1 Evolution in the rest-frame UV luminosity function of UV-continuum selected dropouts over the redshift range 4 < z < 10. The Schechter function parameterizations of the luminosity function are taken from Bouwens et al. (2015b, solid line). The dotted line shows Schechter function parameterizations from the Edinburgh group, at z ∼ 5 from McLure et al. (2009), at z ∼ 6andz ∼ 7fromBowleret al. (2015), at z ∼ 8 from McLure et al. (2013), and at z ∼ 10 from McLeod et al. (2016). The stepwise determinations are shown from several teams (McLure et al. 2009, 2013; Ouchi et al. 2009; Oesch et al. 2013; Bouwens et al. 2015b,c; Bowler et al. 2015; Finkelstein et al. 2015; McLeod et al. 2016). For consistency of comparison, all data points have been adjusted to a cosmology with 0 = 0.3, −1 −1  = 0.7, and H0 = 70 km s Mpc .

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–22.0 –1.4 –2.5 –1.6 –21.5 –3.0 –1.8 M ϕ α UV –21.0 log10 –3.5 –2.0

–20.5 –2.2 –4.0

–2.4 –20.0 –4.5 3 4 5 6 7 8 910 46810 345678910 Redshift Redshift Redshift

Finkelstein et al. (2015) Bouwens et al. (2015b) Bowler et al. (2015)

Figure 2 φ∗,  ,α Evolution in Schechter function parameters ( M UV ) of the rest-frame UV luminosity function of UV-continuum selected dropouts over the redshift range 4 < z < 8. The dashed lines show UV luminosity function Schechter parameter fitting functions derived by Bouwens et al. (2015b).

The redshift evolution of the UV LF over 4 < z < 8(Figure 1) reveals a rapid decline in the abundance of UV luminous galaxies at z > 4. This was first noted following the discovery of the first few z 6−7 dropouts (Stanway et al. 2003, Bouwens et al. 2004b, Bunker et al. 2004, Dickinson et al. 2004) and has since been extended to higher redshifts and confirmed with much larger samples. Current measurements suggest a 15–20-fold decrease in the space density of

galaxies with M UV =−21 in the 900 Myr between z 4andz 8 (McLure et al. 2013, Bouwens et al. 2015b, Finkelstein et al. 2015). In contrast, the space density of galaxies that are ten times

less luminous (M UV =−18.5) decreases by only a factor of 2 to 3 over the same redshift range (Bouwens et al. 2015b, Finkelstein et al. 2015), indicating that low-luminosity galaxies become increasingly dominant contributors to the integrated UV luminosity density at earlier times. As can be seen in Figure 1, the various measurements of the z > 4 UV LF show broad agreement. There do remain some discrepancies, however, particularly at the bright end. Further constraints from wide-area imaging (i.e., VISTA/VIDEO, the VISTA Deep Extragalactic Observations Survey, and Subaru/Hyper Suprime-Cam) and increased attention to small systematics should provide clarity in the coming years. The Schechter parameters describing the luminosity functions at 4 < z < 8 are shown in Figure 2 and are summarized in Table 1. Discrepancies in the Schechter parameters often amplify Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org the actual level of discord between different measurements, owing in large part to the degeneracies in φ, L,andα. This is particularly true at redshifts where the dynamic range in galaxy luminosity is limited. As can be seen in Figure 2, both Bouwens et al. (2015b) and Finkelstein et al. (2015) find that the evolution in the luminosity function at z > 4 can be explained by a rapid decline in the characteristic density of galaxies and a steepening of the faint-end slope. The change in φ causes the decrease in the space density of galaxies seen in Figure 1, and the evolution in α causes the decline to be less rapid at low luminosities. In these studies, the characteristic luminosity is not found to vary strongly with redshift over 4 < z < 8. However, Bowler et al. (2015) argue that the UV LF evolution is partially driven by a change in the characteristic absolute magnitude   . < <  ( M UV 0 4 over 5 z 7). In contrast to the debate over M UV, there now appears to be consensus that α changes at z > 4. The steepening of the faint-end slope was first suggested by Bouwens et al. (2011), albeit with limited significance. Subsequent investigations based on deeper

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Table 1 Schechter parameters for rest-UV luminosity functions of z > 4LBGs Selection φ  −3 −3 α method zphot MUV (10 Mpc ) Bouwens et al. (2015b) − . ± . +0.34 − . ± . B-drop 3.8 20 88 0 08 1.97−0.29 1 64 0 04 − . ± . +0.18 − . ± . V-drop 4.9 21 17 0 12 0.74−0.14 1 76 0 05 − . ± . +0.22 − . ± . i775-drop 5.9 20 94 0 20 0.50−0.16 1 87 0 10 − . ± . +0.21 − . ± . z-drop 6.8 20 87 0 26 0.29−0.12 2 06 0 13 − . ± . +0.23 − . ± . Y-drop 7.9 20 63 0 36 0.21−0.11 2 02 0 23 − . +0.007 − . J-drop 10.4 20 92 (fixed) 0.013−0.005 2 27 (fixed) Finkelstein et al. (2015) − . +0.09 +0.20 − . +0.06 Photo-z 4 20 73−0.09 1.41−0.18 1 56−0.05 − . +0.13 +0.19 − . +0.05 Photo-z 5 20 81−0.12 0.89−0.13 1 67−0.06 − . +0.25 +0.09 − . +0.10 Photo-z 6 21 13−0.31 0.186−0.08 2 02−0.10 − . +0.37 +0.15 − . +0.21 Photo-z 7 21 03−0.50 0.157−0.10 2 03−0.20 − . +0.74 +0.25 − . +0.54 Photo-z 8 20 89−1.08 0.072−0.07 2 36−0.40 McLure et al. (2013), McLeod et al. (2016) − . +0.23 +0.56 − . +0.14 Photo-z 7 19 90−0.28 1.10−0.45 1 90−0.15 − . +0.37 +0.40 − . +0.22 Photo-z 8 20 12−0.48 0.45−0.30 2 02−0.23 − . . +0.05 − . Photo-z 9 20 1 (fixed) 0 24−0.05 2 02 (fixed) Photo-z 9 −19.65 ± 0.15 0.45 (fixed) −2.02 (fixed) − . . +0.05 − . Photo-z 10 20 1 (fixed) 0 13−0.04 2 02 (fixed) Photo-z 10 −19.41 ± 0.17 0.45 (fixed) −2.02 (fixed)

imaging have confirmed this trend (McLure et al. 2013, Schenker et al. 2013b). Redshift evolution in α is now established at greater than 3σ ,varyingfromα −1.6atz 4toα −2.1atz 7 (Bouwens et al. 2015b, Finkelstein et al. 2015). The evolution of the Schechter parameters at 4 < z < 8 have been summarized by simple fitting functions (e.g., Bouwens et al. 2008, 2015b), allowing the UV LF to be estimated at arbitrary redshifts. Assuming the Schechter parameters vary linearly with redshift, Bouwens et al. (2015b) find that  = − . ± . + . ± . − , M UV ( 20 95 0 10) (0 01 0 06)(z 6) (3)

 +0.11 (−0.27±0.05)(z−6) −3 −3 φ = (0.47− . )10 10 Mpc , (4)

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α = (−1.87 ± 0.05) + (−0.10 ± 0.03)(z − 6). (5) The fitting functions are particularly valuable for understanding the relative space density of LBGs and LAEs. However caution must be exercised when extending the LFs derived from the fitting functions to redshifts and luminosities that are not covered by current surveys. As is discussed in Section 3.2, there are some indications that the evolution in the range 8 < z < 10 is more rapid than would be expected from the evolution at 4 < z < 8. The basic evolutionary trends suggested by the above fitting functions are easily reproduced with theoretical models. A commonly used approach is to predict UV LF from the halo mass func- tion by applying a mass-to-light ratio or a simple star formation prescription (e.g., Mashian et al. 2016, Mason et al. 2015, Sun & Furlanetto 2016). Predictions from simulations and semiempirical

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models predict faint-end slope evolution that is consistent with current observational constraints (e.g., Trenti et al. 2010, Jaacks et al. 2012, Tacchella et al. 2013), reflecting the steepening of the halo mass function. The latest UV LF measurements suggest that higher-redshift ha- −1.5 los are more UV luminous per unit mass. Bouwens et al. (2015b) find that M halo/LUV ∝ (1+z) provides the best fit to the UV LF evolution over 4 < z < 8, matching the scaling of the dynamical time with redshift. The underlying mass function is steeper than the faint-end slope of the UV LF,

suggesting that the M halo/LUV ratio is larger in low-mass halos (due to, e.g., supernovae feedback). The evolution in the UV LF has also been used to constrain the likely star formation histories of individual galaxies (e.g., Bouwens et al. 2008, Stark et al. 2009). The most recent efforts have exploited number density matching techniques, demonstrating that rising star formation histories are preferred for galaxies at z > 4 (e.g., Papovich et al. 2011). The dynamic range probed by z > 6 datasets is now beginning to allow the first investigations into whether the UV LF departs from the form suggested by a Schechter function. In particular, at the bright end, various groups have considered whether the data require the steep exponential decline that is characteristic of a Schechter function. The sharp UV luminosity cutoff at the bright end is thought to be regulated by a combination of physical processes, including the inefficiency of gas cooling in massive halos, (AGN) heating, and the increased dust content of the most luminous galaxies (see Bouwens et al. 2008). At very high redshifts, these processes may not yet be effective in the dark matter halos that are present. If this is the case already at z 6−7, the bright end of the UV LF may not exhibit an exponential cutoff as dictated by the Schechter function. The first claims of possible evolution in the bright-end slope at very high redshift have appeared recently, following the large investment in wide-area imaging surveys from the ground (Bowler

et al. 2014, 2015). These studies have argued that the space density of UV luminous (M UV = −22.75 to −21.75) z 7 galaxies identified in UltraVISTA and the UDS is inconsistent with the exponential cutoff of the Schechter function. A double power law (DPL) with a shallower decline at the bright end is found to provide a better fit to the LF data. The DPL function is defined as follows: φ φ(M) =   , (6) 100.4(α+1)(M −M ) + 100.4(β+1)(M −M ) where φ, M ,andα are the characteristic normalization, luminosity and faint-end slope, and β is the power-law slope at the bright end. Bowler et al. (2014) find that the bright end of the z 7 β =− . +0.4 UV LF can be fit with a power law slope of 4 6−0.5.Atz 6, a DPL fit reveals a steeper β =− . +0.5 slope ( 5 1−0.6) and is equally well fit by a Schechter function. If this result is confirmed, it would suggest that the physical mechanism (e.g., dust obscuration or feedback) that introduces the exponential cutoff in the abundance of bright galaxies becomes effective between z 7and

Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org z 5. There is not yet complete consensus on this conclusion, however. Bouwens et al. (2015b) and Finkelstein et al. (2015) have included the new ground-based constraints from Bowler et al. (2014) in their most recent analysis and find that the exponential cutoff provides a slightly better fit to their data at z 7. They both argue that there remains no compelling evidence requiring departure from a Schechter function at z 6−7. The tension between different groups should ease in the future following new imaging surveys soon to be under way. There is also likely to be departure from a Schechter function at the faint end. Star formation is likely to be suppressed in low-mass halos, causing the UV LF to turn over at very low luminosities. The physical processes responsible for the turnover could include a combination of factors, in- cluding suppression of molecular hydrogen formation at low metallicity and photo-heating from the UV background (see, e.g., Kuhlen & Faucher-Giguere` 2012, Kuhlen et al. 2013). The total ionizing output of early galaxies, and hence their contribution to reionization, can only be derived

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if the turnover luminosity is known. The deepest blank field studies at z 6 suggest the UV LF

shows no sign of a turnover to M UV −17. Early constraints on the number of highly magnified, gravitationally lensed galaxies from the HFF initiative suggest that counts rise to M UV −15 at z 7 (e.g., Atek et al. 2015a, Ishigaki et al. 2015, Livermore et al. 2016).

3.2. The Census of z 9–10 UV-Selected Dropouts Over the past five years, roughly 20 z 9−11 candidates have been identified in deep WFC3/IR imaging of the HUDF and CANDELS fields (Oesch et al. 2010, 2012, 2013, 2014, 2016; Ellis et al. 2013; McLure et al. 2013; Bouwens et al. 2015b,c). These samples have been augmented by several bright gravitationally lensed systems from the CLASH survey (Zheng et al. 2012, Coe et al. 2013, Bouwens et al. 2014a) and a combination of lensed and unlensed galaxy found in the HFF initiative (Zitrin et al. 2014; Atek et al. 2015b; Ishigaki et al. 2015; McLeod et al. 2015, 2016; Oesch et al. 2015a). Not surprisingly given the small number of candidates and limited area of the surveys, there have been differences in the measured UV LF evolution at 8 < z < 10. Several studies have argued that the decline in the UV luminosity density is accelerated at z > 8 (Oesch et al. 2012, 2013, 2014; Bouwens et al. 2014a, 2015b). Bouwens et al. (2015c) find that the volume densities of bright galaxies at z 9andz 10 are respectively 4.5× and 8× lower than that at z 8. These luminosity densities are 2× lower than expected based on extrapolation of the smooth evolution observed over 4 < z < 8. Such accelerated evolution is consistent with predictions from some hydrodynamical simulations and semianalytic models (e.g., Finlator et al. 2011, Dayal et al. 2013, Jaacks et al. 2013, Tacchella et al. 2013; cf. Sun & Furlanetto 2016), suggesting that observations of galaxies at fixed luminosity may begin to probe a steeper part of the underlying halo mass function at z > 8. However, other observational studies report that evolution over 8 < z < 10 continues at the same rate suggested by 4 < z < 8 galaxies (Coe et al. 2013; Ellis et al. 2013; McLeod et al. 2015, 2016). HST promises to provide further insight into this area in the coming years, thanks to new parallel observations from the BoRG campaign and a very large initiative (RELICS, the Reionization Lensing Cluster Survey) to search for bright lensed galaxies in 39 cluster fields.

3.3. The Lyman-α Luminosity Function The large LAE surveys conducted in the past decade have led to improved measurements of the luminosity function of LAEs at z 6 and beyond. Because of the resonant interaction between Lyα and neutral hydrogen, the census of LAEs at any redshift is very sensitive to the transmission of Lyα through both galaxies and the IGM. Changes in the neutral hydrogen distribution or dust

Access provided by California Institute of Technology on 01/11/17. For personal use only. α Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org content of galaxies can thus drive rapid evolution in the observed space density of Ly emitting galaxies. As observations enter the reionization era, the evolving IGM neutral hydrogen content will also modulate the counts of Lyα emitters (Miralda-Escude´ 1998). For these reasons, the evolution in the LAE LF is not expected to track that of the UV-continuum selected dropouts. In this section, we focus on the census of LAEs at z 6, after reionization is completed, and we try to understand the relative evolution of LAEs and LBGs over 3 < z < 6. We revisit the evolution of the LAE LF in Section 5.1 when discussing the use of Lyα as a probe of the IGM. The luminosity function of LAEs at z = 5.7isshowninFigure 3a, and the Schechter pa- rameters from three different LAE LF determinations are summarized in Table 2. As can be seen in the table, there is good agreement between the Kashikawa et al. (2011) and Ouchi et al. (2008) luminosity functions at z = 5.7, derived in the SDF and SXDS, respectively. Both studies predict LAE space densities that are roughly 2.5× larger than derived in Hu et al. (2010). The

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ab10–2 ) 10–2 –3

z = 3.1 –3 Mpc 10 –1 z = 5.7 10–4 ] –3 –3 10 10–5 LBG z~3.1 z Mpc LAE ~3.1

–1 –6

Number (mag 10 –22.5 –22.0 –21.5 –21.0 –20.5 –20.0 –19.5 = 1)

L M UV 10–4 log ) –3

Δ 10 –3 [( n

Mpc –4

–1 10

–5 10 z = 6.6 10–5 LBG z~3.7 42.0 43.0 44.0 LAE z~5.7 L α –1 –6

log (Ly ) (erg s ) Number (mag 10 –22.5 –22.0 –21.5 –21.0 –20.5 –20.0 –19.5 M UV

Figure 3 (a) Evolution in Lyman-α emitting galaxy (LAE) luminosity function over redshifts 3.1 < z < 6.6 as shown in Ouchi et al. (2010). Little evolution is seen over 3.1 < z < 5.7, in contrast to the trend seen in the Lyman break galaxy (LBG) population. Blue and red filled circles give the Lyman-α luminosity function data points at z = 5.7andz = 6.6 from Ouchi et al. (2008) and Ouchi et al. (2010), respectively. Open symbols show the z = 6.6 LAE luminosity function determined in individual survey fields, demonstrating significant field-to-field variance. (b) Comparison of UV luminosity functions of LAEs and LBGs at z 3.1andz 5.7. The UV luminosity function of LAEs (blue circles) is from Ouchi et al. (2008), and the LBG UV luminosity functions (red triangles)atz = 3.1andz = 5.7are from Reddy & Steidel (2009) and Bouwens et al. (2015b), respectively. The relative space density of LAEs and LBGs increases toward higher redshift over 3.1 < z < 5.7.

Table 2 Schechter parameters for LAE luminosity functions  −  − − α 42 1 φ 4 3 α zLy LLyα (10 erg s ) (10 Mpc ) Reference +3.0 +7.4 − . 5.7 6.8−2.1 7.7−3.9 1 5 (fixed) Ouchi et al. (2008) +8.2 +3.5 − . 5.7 9.5−3.1 3.6−2.5 2 0 (fixed) Ouchi et al. (2008) +0.6 +3.0 − . 6.6 4.4−0.6 8.5−2.2 1 5 (fixed) Ouchi et al. (2010) +9.9 . ± . − . 5.7 10.0−3.6 1 1 0 2 1 5 (fixed) Hu et al. (2010)

Access provided by California Institute of Technology on 01/11/17. For personal use only. + . Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org 29 8 . ± . − . 6.5 10.0−5.0 0 6 0 2 1 5 (fixed) Hu et al. (2010) +34.3 . ± . − . 5.7 15.8−7.9 0 5 0 1 2 0 (fixed) Hu et al. (2010) +187 . ± . − . 6.5 12.6−7.6 0 3 0 1 2 0 (fixed) Hu et al. (2010) +1.6 +0.6 − . 5.7 10.5−1.4 2.8−0.6 1 5 (fixed) Kashikawa et al. (2011) +1.5 +3.1 − . 6.5 5.8−1.2 5.2−1.4 1 5 (fixed) Kashikawa et al. (2011) +2.0 +0.3 − . 5.7 13.2−1.2 1.8−0.4 1 7 (fixed) Kashikawa et al. (2011) +1.7 +2.6 − . 6.5 6.6−5.2 4.0−1.4 1 7 (fixed) Kashikawa et al. (2011) +8.0 +17.6 − . 7.3 2.7−1.2 3.7−3.3 1 5 (fixed) Konno et al. (2014)

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discrepancy is most significant at lower Lyα luminosities (0.6−1.0×1043 erg s−1). Future ultradeep spectroscopy of faint z = 5.7 LAE candidates should be able to resolve the discrepancy. As can be seen in Table 2, none of the narrowband imaging surveys described above are able to constrain the faint-end slope. Schechter parameters are instead provided for a variety of fixed values for α. The multislit narrowband surveys have recently begun to shed some light on this issue. Dressler et al. (2015) argue that the counts of faint LAEs support a very steep faint-end slope (−1.95 <α<−2.35), consistent with the values that are derived from the z 5.9 i -band dropout samples. Comparison of the z = 5.7 LAE LF to that at z = 3.1 reveals little to no evolution, indicating that the space density of LAEs remains constant over 3.1 < z < 5.7, a period spanning more than one billion years of cosmic history. Over this same period, the LBG luminosity function evolves

strongly, with the volume density of bright LBGs (M UV =−21) dropping by a factor of five between z 3.1andz 5.7 (according to the Bouwens et al. 2015b fitting functions described in Section 3.1). The different evolutionary trends can be explained if the transmission of Lyα through galaxies increases between z 3andz 6, counteracting the decline in the number density of the underlying galaxy population. For this to occur, the conditions within galaxies at z 5−6 must be more conducive to the escape of Lyα. Such evolution could be driven by the reduced dust content (see Section 4.1) and lower covering fraction of hydrogen (see Section 4.3) in galaxies at earlier times. The UV continuum LF of LAEs provides a more direct comparison to the LBG population (see Figure 3b). At z 3, Ouchi et al. (2008) find that the volume density of LAEs is only 10% that

of LBGs for galaxies with continuum luminosities in the range −22 < M UV < −20. Taken at face value, this implies that roughly one in ten luminous z 3 galaxies have Lyα EWs comparable to those in narrowband samples (>40–60 A˚ for z = 3.1 LAEs in Ouchi et al. 2008), fully consistent with the fraction of LBGs with large EW Lyα emission: At z 3, only 2(25)% of LBGs have rest-frame Lyα EWs greater than 100(20) A˚ (Shapley et al. 2003). Between z = 3.1andz = 5.7, the relative abundance of LAEs and LBGs changes significantly (Figure 3b). At z 5.7, the

volume density of LAEs with M UV =−21 is roughly 25% that of LBGs at the same redshift, suggesting that the fraction of LBGs with large EW Lyα emission increases by roughly 2.5× over 3.1 < z < 5.7. The comparison of the LAE and LBG census at 3 < z < 6 demonstrates that galaxies evolve in such a way to allow Lyα to become more common in galaxies at earlier times.

3.4. The Contribution of Dusty Star-Forming Galaxies The census of massive dust-obscured galaxies within the first billion years remains incomplete. Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org The Herschel/SPIRE color selection utilized by Riechers et al. (2013) identified five dusty galaxy candidates over 21 deg2. Because not all of these are likely to be at z > 6, they place an upper −2 limit on the source density ( ≤0.24 deg ) of massive starbursts at z > 6 with S500µm > 30 mJy. The confirmation of one of the five candidates at z = 6.34 suggests a surface density of greater than 0.05 arcmin−2. For comparison, the surface density of z 6 LBGs in the UDF reaches above 3arcmin−2, which is orders of magnitude greater than the current upper limit on massive dust obscured galaxies. The longer-wavelength selection conducted with SPT provides an efficient method of identifying the highest-redshift dusty star-forming galaxies. Now that the redshift distribution of the sample has been established (Strandet et al. 2016), the next step is to calculate the evolving space density of the population over 4 < z < 7. Such a calculation is not trivial because the effective survey volume is modulated by magnification from gravitational lensing, but the value would provide new insight into the assembly of massive galaxies at very early times.

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4. EARLY STAR-FORMING GALAXY PROPERTIES The past decade has also witnessed the first important steps toward the characterization of the detailed nature of galaxies at very high redshifts. Galaxies within the first billion years are likely to be considerably different from the well-studied population at z 2−3, with younger and more metal-poor stars, reduced dust content, lower stellar masses, and smaller physical sizes. This section provides a review of the physical picture that is now emerging.

4.1. UV Continuum Slopes The observed UV continuum slope of a star-forming galaxy is primarily regulated by the nature of its massive stars (i.e., metallicity, age, multiplicity) and the dust that reddens the starlight as it β exits the galaxy. Typically parameterized as β (where flux density fλ λ ), the UV slope of a z > 4 galaxy can be estimated from multiwavelength HST broadband imaging. Not only can UV slopes provide constraints on dust extinction (necessary for converting the UV luminosity to SFR), but they offer a glimpse of the stellar populations in place within the first billion years. Over the past ten years, considerable effort has been invested in the characterization of the UV colors of galaxies at z > 4. One of the key goals is determining whether continuum slopes evolve with redshift, as may be expected if the metal and dust content is reduced at very high redshift. If a relationship between metallicity and galaxy luminosity is established at early times, one may also see a trend

between β and M UV. The procedure for determining UV slopes typically involves fitting the flux densities from broadband to a power law. Generally filters that may be contaminated by the Lyα emission line or Lyα forest absorption are not used in the calculation. It has become customary to determine transformations between observed rest-frame UV color and β for a given dropout sample. Such transformations are derived by integrating population synthesis models (for which the effective UV slope is known) through the relevant filter transmission curves to determine the rest-frame UV colors at redshifts sampled by the dropout population of interest. Comparison of the effective UV slopes and rest-frame UV colors then yields the desired relation. For the z-band dropout population at z 7, Bouwens et al. (2014b) derive the following mapping between

J125−H160 galaxy colors and UV slope: β =−2.0 + 4.39( J125−H160). The 4.39 prefactor in this relation is within 1–2% of that used in other studies (Bouwens et al. 2010, Dunlop et al. 2012). By definition, galaxies with flat spectral slopes in fν (and hence zero color in the AB magnitude system) will have β =−2. The first UV slope measurements of z 6 galaxies appeared a decade ago (Bouwens et al. 2003, 2006; Stanway et al. 2005). Using deep NICMOS imaging, it was found that the typical

Access provided by California Institute of Technology on 01/11/17. For personal use only. β − − . = Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org UV slopes ( 2to 2 2) at z 6 were considerably bluer than galaxies at z 3. Because these UV slopes are very similar to the intrinsic values of population synthesis models (assuming solar metallicity and ages above 100 Myr), it was argued that the stellar continuum of typical UV- selected galaxies at z 6 suffers very little reddening from dust. Bouwens et al. (2009) built on these studies, providing the first comprehensive investigation of the relationship between UV slope and luminosity in continuum-selected dropouts over z 2−6. The results revealed a correlation

between β and M UV and suggested that there was likely a shift toward bluer colors at higher redshift. But measurement of UV slopes in all but the brightest galaxies at z > 5 remained difficult

with NICMOS imaging. Even with 10σ detections in J125 and H160, the UV slope measurement is fairly uncertain (β 0.8). The emergence of deep WFC3/IR imaging in 2009 greatly improved the fidelity of UV slope measurements, leading to a flurry of detailed investigations (Bouwens et al. 2010, 2012, 2014b;

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Wilkins et al. 2011; Dunlop et al. 2012, 2013; Finkelstein et al. 2012; Rogers et al. 2013, 2014). There has been general agreement that z > 6 galaxies are blue (β ≤−2), confirming the earlier NICMOS results. But there have also been small yet distinct differences in the average β values reported by the different papers, leading to a debate regarding the extent to which UV slopes vary with luminosity and redshift. It is likely that the discrepancies stem largely from minor differences

in the method of photometry. At very high redshift, the short lever arm provided by the J125

and H160 measurements dictates that the β measurement will be very sensitive to systematics in the measured colors. It can be seen from the mapping between UV color and β that very small

(<0.05 mag) systematics in the ( J125−H160) color measurement will shift the UV slopes by up to β = 0.22, comparable to the offset found between the different studies. Such small systematics can arise owing to errors in the point spread function (PSF) used for aperture matched photometry and the application of fixed aperture photometry to resolved galaxies (see discussion in Bouwens et al. 2014b). Additional luminosity-dependent biases can affect the inferred UV slope distributions through the inclusion of sources with nonrobust photometry, or through use of filters that are contaminated by Lyα emission and IGM absorption. Perhaps the most heated debate has surrounded the luminosity dependence of the UV slope. At redshifts 4 < z < 6, there is now emerging consensus that a strong color-magnitude relationship exists. Rogers et al. (2014) have analyzed the UV slopes of 584 z 5 LBGs spanning a factor of

100 in luminosity. The results strongly support a relationship between UV slope and M UV, with the lowest-luminosity galaxies exhibiting the bluest colors. A simple linear fit results in a slope of

dβ/dM =−0.12 ± 0.02 and a zero-point of β(M 1500 =−19.5) =−1.93 ± 0.03. This is very close to the relationship derived by Bouwens et al. (2014b) and is consistent with the binned data points in Finkelstein et al. (2012). There is less consensus in the literature as to whether there

is a similar β-M UV relationship at z > 6, with several studies arguing in favor of a relationship (Bouwens et al. 2010, 2012, 2014b; Wilkins et al. 2011) and others finding no evidence for any

dependence of β on M UV at the highest redshifts (Dunlop et al. 2012, Finkelstein et al. 2012, Rogers et al. 2013). In one of the largest studies to date, Bouwens et al. (2014b) measured the UV slopes of 184 z 7 galaxies discovered with deep WFC3/IR imaging (see Figure 4a), finding a

significant correlation between M UV and β,

β = (−2.05 ± 0.09 ± 0.13) + (−0.2 ± 0.07)(M UV + 19.5). (7)

The slope of the β-M UV relationship is consistent with that at z 4−5 within the uncertainties. Bouwens et al. (2014b) investigate systematics facing earlier studies (including their own) and find that the above relationship is consistent with all previous studies after accounting for these

Access provided by California Institute of Technology on 01/11/17. For personal use only. β Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org systematics. There is some hint that the relationship between and M UV flattens at luminosities

lower than M UV =−19 at 4 < z < 6 (Bouwens et al. 2014b), potentially revealing differences in the nature of the faintest objects. However, UV slopes remain incredibly difficult to measure at these low luminosities, and indeed there is no consensus that such a flattening is required to fit

the β-M UV relationship (Rogers et al. 2014). The WFC3/IR measurements reveal that galaxies at z = 7 are bluer than those of similar luminosity at z 2−3, confirming earlier studies with NICMOS. Over the more restricted redshift range 4 < z < 7, most groups have identified mild but significant evolution in the average UV slope at fixed luminosity (Wilkins et al. 2011, Finkelstein et al. 2012, Bouwens et al. 2014b). Bouwens

et al. (2014b) considered biweight mean UV slopes in the range −19.3 < M UV < −16.7for galaxy samples at z 4−7, finding that β evolves by −0.10 ± 0.05 per unit redshift, consistent with several previous studies (Bouwens et al. 2012, Finkelstein et al. 2012). The best fit relation

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Age of the Universe (Gyr) ab5 4 3 2 1 0 2.0 Salmon et al. (2015) L* z~7 z = 3 Somerville et al. (2012) SAM z = 4 Relation 1.5 Literature z > 2 –1 z = 7 Best fit Davé et al. (2011) Hydro Sim

) Neistein & Dekel (2008) 1.0 –1 –2 β 0.5

–3 Bouwens et al. (2014b) 0.0 Wilkins et al. (2011) log(sSFR/Gyr Bouwens et al. (2012) –0.5 –4 Finkelstein et al. (2012) Dunlop et al. (2013) –1.0 –22 –20 –18 1 2 3 4 5 6 7 M UV,AB Redshift

Figure 4

(a) Dependence of the UV continuum slope β on M UV at z 7, adapted with permission from Bouwens et al. (2014b). (b) Redshift evolution in specific star formation rate (sSFR) of galaxies with fixed stellar mass, reproduced from Salmon et al. (2015) with measurements from literature included from literature as white squares. Figure courtesy of B. Salmon. Other abbreviations: Hydro Sim, hydrodynamical simulation; SAM, semianalytical model.

derived from this calculation is

β =−2.14 ± 0.05 − (0.10 ± 0.05)(z − 4.9), (8)

clearly demonstrating the small shift toward bluer colors at fixed M UV in the colors at z > 4. The first reliable constraints on the mean UV slopes at z 8 − 9 emerged following the

addition of the deep JH140 imaging in the UDF (e.g., Dunlop et al. 2013, Bouwens et al. 2014b).

The ( JH140−H160) color provides a clean measurement of the UV slope at z 8−8.5astheJH140 filter is free of Lyα emission and IGM absorption at z < 9. Following the approach used at z 7,

Bouwens et al. (2014b) have derived a mapping between the ( JH140−H160) color and UV slope,

β =−2.0 + 8.98( JH140 − H160). Owing to the very small wavelength baseline probed by the (JH140−H160) color, systematics and uncertainties in the color measurement will translate into very large errors in the UV slope measurement. Current measurements suggest mean UV slopes between β =−2.3andβ =−1.9atz 8 (Finkelstein et al. 2012, Dunlop et al. 2013, Bouwens et al. 2014b), revealing no strong evolution from z 4. Attempts to extend these measurements

Access provided by California Institute of Technology on 01/11/17. For personal use only. . − β − Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org to the brightest z 8 5 9 galaxies also point to UV slopes that are not much bluer than 2 (Dunlop et al. 2013, Bouwens et al. 2014b). Uncertainties are sizeable at z 8, typically in the range β 0.3−0.5. More secure measurements of UV slopes at z 8−10 will emerge in several years from deeper JWST imaging.

The relationship between UV slope and M UV is consistent with theoretical predictions (Finlator et al. 2011, Dayal et al. 2013), likely reflecting variations in the mean metallicity and dust content with the stellar mass and UV luminosity (Bouwens et al. 2009, Finkelstein et al. 2012). Existing measurements provide no evidence for unusual stellar populations with extremely blue UV slopes (β −3), as might be expected if the UV stellar continuum at z 7 is dominated by very young and metal-deficient stars. Establishing anything further about stellar populations is difficult, as it is challenging to relate the β distributions directly to constraints on the massive stars because the metallicity, dust content, and mean stellar age all affect the UV slope. Ultimately

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spectroscopy will be required to provide more insight into the nature of massive stars in early galaxies.

4.2. Stellar Masses and Specific Star Formation Rates Hubble imaging probes the rest-frame UV of z > 6 galaxies, measuring the light produced by O and B stars. Although the UV continuum luminosity provides an estimate of the ongoing star formation activity, it does not constrain the past history of growth. Knowledge of the light provided by old stars can only be achieved by sampling the SED at wavelengths longer than the age-sensitive Balmer break. For galaxies in the reionization era, this requires photometric constraints at observed wavelengths of 3–5 µm. The deep IRAC [3.6] and [4.5] imaging that was obtained following the launch of Spitzer in 2003 provided the first sensitive constraints on the flux density of reionization-era galaxies at 3–5 µm. Comparison of observed broadband SEDs with HST and Spitzer/IRAC photometry to synthesis models allows constraints to be placed on the stellar mass and specific star formation rate (sSFR), illuminating the physical processes that govern the growth of early galaxies and provide a fossil record of star formation that can be used to assess the contribution of galaxies to reionization. Over the past decade, deep Spitzer/IRAC imaging has been obtained for many of the extra- galactic deep fields described in Section 2. Typical 5σ IRAC [3.6] and [4.5] magnitudes are in

the range mAB 25.1–26.5 for the CANDELS area (Bouwens et al. 2015b). Given that HST near-IR imaging typically reaches 3–4 magnitudes deeper than IRAC, current analysis is generally limited to relatively bright systems. There are several additional challenges in the exploitation of the Spitzer imaging datasets. First, the relatively broad PSF of Spitzer leads to confusion with foreground galaxies, requiring careful application of deconfusion algorithms (for details see Labbe´ et al. 2015). This can lead to considerable uncertainty in the flux of faint sources with projected locations very close to bright galaxies. A problem is the contamination of the [3.6] and [4.5] filters by rest-frame optical nebular emission lines (i.e., [OIII]λ5007, Hα). If the emission lines have very large EWs, they can contribute significantly to the broadband flux densities, making it difficult to obtain clean measurements of the rest-optical stellar continuum and resulting in consid- erable uncertainty in the stellar mass and sSFR (e.g., Schaerer & de Barros 2009). One method of disentangling the nebular contribution is to fit galaxies with population synthesis models equipped with predictions of nebular line emission EWs (e.g., de Barros et al. 2014). An alternative approach is to study galaxies in specific redshift intervals that ensure that one of the IRAC bandpasses is free of strong emission lines (Shim et al. 2011, Stark et al. 2013, Smit et al. 2014). If optical lines are very prominent, contaminated filters will appear to have a flux excess relative to adjacent filters that are uncontaminated. The amplitude of this flux excess can be used to estimate the total EW Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org of contaminating emission lines. The first IRAC detections of z 6−7 star-forming galaxies emerged shortly after the launch of Spitzer (Egami et al. 2005, Eyles et al. 2005, Labbe´ et al. 2006). Stark et al. (2009) built on these initial studies by conducting an analysis of the stellar mass distribution in a large sample of galaxies spanning the redshift range 4 < z < 6. This work was subsequently extended to z 7(e.g., Finkelstein et al. 2010, Labbe´ et al. 2010, McLure et al. 2010, Gonzalez´ et al. 2011), z = 8(Labbe´ et al. 2013), and z 9−10 (Oesch et al. 2014) following the installation of WFC3 on HST. In the past year, the influx of bright targets from CANDELS has culminated in several comprehensive investigations of the stellar mass and sSFR of galaxies at z 4−7 (Duncan et al. 2014, Gonzalez´ et al. 2014, Grazian et al. 2015, Salmon et al. 2015). One of the primary goals of such investigations is to characterize the evolution in the stellar mass–SFR main sequence in the first two billion years. Simple theoretical expectations predict that

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3 z~4 z~5 z~6

)] 2 –1 yr ☉

1 log[SFR/(M

0 Medians

8.5 9.0 9.5 10.0 10.5 8.5 9.0 9.5 10.0 10.5 8.5 9.0 9.5 10.0 10.5 11.0

log(mass/M☉) log(mass/M☉) log(mass/M☉)

Figure 5 Relationship of stellar mass and star formation rate (SFR) over redshifts 4 < z < 6, adapted with permission from Salmon et al. (2015). The yellow circles show the median SFR in bins of stellar mass. The blue, green, and orange pixels demonstrate the range of SFRs and stellar masses spanned at z 4, 5, and 6, respectively. The darker-shaded pixels indicate a larger number of objects. A distinct star formation rate (SFR)–stellar mass relationship is apparent in each redshift bin.

the normalization of the main sequence should track the specific mass inflow rate of , which is thought to increase with redshift as M˙ /M (1 + z)2.25 (e.g., Dave´ et al. 2011). In this picture, the average sSFR for galaxies of fixed stellar mass should increase by nearly 10× over 2 < z < 7. The first evidence for a relationship between SFR and stellar mass at z > 4 appeared following the

demonstration that the stellar mass increased with M UV in galaxies at z 4−6 (Stark et al. 2009).  These results revealed that the M -M UV relationship changed very slowly with redshift over 4 < z < 6, suggesting mild evolution in the SFR-M  main sequence. Using these results, along with new constraints at z 7, Gonzalez´ et al. (2010) argued that the sSFR appeared to be constant −1 9 (sSFR = 2 Gyr ) in galaxies with stellar mass 10 M over 2 < z < 7. This purported plateau in sSFR is very different from the theoretical picture discussed above, motivating many investigations into what might be suppressing star formation at early times. But these early results did not take into account the contribution of nebular emission to the Spitzer bandpasses. Corrections for this emission tend to reduce the stellar masses and increase the sSFR at z 5−7 (e.g., Stark et al. 2013, de Barros et al. 2014). The stellar mass studies made possible by CANDELS have improved the reliability of stellar mass and sSFR determinations, leading to clear signatures of a galaxy stellar

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Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org mass–SFR main sequence (Figure 5)atz 6. Salmon et al. (2015) find that the main sequence can be parameterized as SFR M α where α = 0.54 ± 0.16 at z 6andα = 0.70 ± 0.21 at z 4. The intrinsic scatter in SFR at fixed stellar mass is found to be small (0.2–0.3 dex) for galaxies 9 with stellar masses greater than 10 M (Salmon et al. 2015), suggesting that the net gas inflow rate is relatively smooth on timescales probed by the UV SFRs (100 Myr). Although the level of nebular contamination remains a source of uncertainty at z 5−7, most studies now support a smoothly rising sSFR at fixed stellar mass (Stark et al. 2013, Duncan et al. 2014, Salmon et al. 2015), with the average sSFR reaching large values (0.5–1 Gyr−1)byz 7, similar to the values seen in semianalytic models and simulations (Figure 4b). Additional support for large sSFRs at z 6−7 comes from the EW of rest-optical emission lines. If the sSFRs are very large, then the emission lines should be very prominent against the weak rest- optical stellar continuum, leading to large flux excesses in IRAC contaminated bandpasses. In the

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redshift range z 6.6−6.9, the [OIII]+Hβ lines lie in the [3.6] filter, and the [4.5] filter is clean of strong emission lines. In a series of papers (Smit et al. 2014, 2015), it has been shown that galaxies in the range 6.6 < z < 6.9 have [3.6]–[4.5] colors that demonstrate that [OIII]+Hβ emission is contributing greatly to the flux density in the 3.6 µm filter, revealing extreme equivalent widths (1000–3000 A˚ rest frame). Only when galaxies are dominated by very young stellar populations will such prominent optical lines be present. This work has recently been extended to even higher redshifts in a study by Roberts-Borsani et al. (2016), exploiting a window at 7 < z < 9inwhich [OIII]+Hβ is in the [4.5] filter and no comparably strong line falls in the [3.6] filter. Four such extreme optical line emitters were identified in the CANDELS fields, all of which have now been spectroscopically confirmed via Lyα detections. Intriguingly, these studies suggest that perhaps up to 50% of the galaxy population at z 6.6−9 is in such an extreme emission line phase (Smit et al. 2014, 2015; Roberts-Borsani et al. 2016). This not only supports the notion that the average sSFR is very large at z > 6, but also has significant implications for future spectroscopic studies with JWST. If rest-optical lines are as prominent as implied by Spitzer/IRAC flux excesses, galaxies should be very easy to target spectroscopically with the Near-Infrared Spectrograph (NIRSpec) aboard JWST. The second main objective of efforts to characterize SEDs is measurement of the stellar mass function (SMF) and the integrated stellar mass density (SMD). Because galaxies are generally selected in the rest-frame UV at z > 4, existing measurements will miss any galaxies that are quiescent. Given the difficulty of identifying faint sources and characterizing incompleteness in the IRAC images, most initial measurements determined the stellar mass function by a simple  application of the M -M UV relationship to the UV luminosity functions (e.g., Gonzalez´ et al. 2011, Lee et al. 2012, Stark et al. 2013, Song et al. 2016a). Although this is a logical approach given the

limitations of the current datasets, the slope and scatter in the stellar mass–M UV relationship lends considerable uncertainty to stellar mass functions determined in this manner. This is particularly true at the faint end, where Spitzer/IRAC measurements become increasingly unreliable. Recently,

Duncan et al. (2014) derived the SMF using a traditional 1/V max method on a sample of dropouts in the CANDELS GOODS South field. The resulting SMF reveals a steep faint-end slope (α −1.9) at z 4−7, close to that of the UV LF. This value is steeper than the earlier studies based on the  M -M UV relationship, and closer to that expected in simulations and semianalytic models (e.g., Duncan et al. 2014). In several years, deep JWST/Near Infrared Camera (NIRCam) will provide much deeper images at 2–5 µm, enabling the form of the SMF to be examined in far greater detail than is allowed by current datasets.

4.3. Insight into Massive Stellar Populations, Neutral Gas Covering Fractions, α Access provided by California Institute of Technology on 01/11/17. For personal use only. and Lyman- Velocity Offsets from Rest-Frame UV Spectroscopy Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org Over the past several years, rest-frame UV spectroscopy has begun to make meaningful contribu- tions to our understanding of the nature of early galaxies. Insight into the properties of large-scale gaseous outflows has emerged from the large spectral databases at z 4−6 (e.g., Vanzella et al. 2009, Stark et al. 2010). Because the majority of individual galaxies at z > 4 are much too faint for absorption line analysis, most knowledge has been gained through stacking analyses. Jones et al. (2012) created composite rest-UV spectra at z 4andz 5 from a spectroscopic sam- ple of 81 LBGs. The strength of low-ionization absorption lines (which trace neutral gas) were found to decrease toward earlier times. Because of the low resolution of the composite spectra, it was impossible to determine whether the evolution in the absorption lines was due to a lower covering fraction of neutral gas or changes in the outflow kinematics. Jones et al. (2013) consid- ered this issue in more detail through analysis of high-quality Keck spectra of three very bright

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gravitationally lensed galaxies at z 4. The higher resolution of the spectra reveals definitively that weaker low-ionization absorption lines arise from a smaller covering fraction of neutral gas in this small sample of galaxies. Such a decrease in the covering fraction of the neutral gas over 3 < z < 5 should allow a larger fraction of the Lyα luminosity to escape at earlier times, consistent with the evolution of Lyα emitters found over this redshift interval (Ouchi et al. 2008, Stark et al. 2010, 2011; see discussion in Section 3.3). These results are consistent with the very low-HI columns implied by GRB absorption line spectroscopy at z 6 (e.g., Chornock et al. 2014). The tentative indications of covering fraction evolution have significant implications for the contribution of early galaxies to reionization, as a patchy distribution of neutral hydrogen will also allow a larger fraction of ionizing radiation to escape into the IGM (Shapley et al. 2003, Jones et al. 2013). Given the difficulty in directly constraining the ionizing escape fraction in star-forming galaxies at z > 4, efforts to extend the analysis of Jones et al. (2013) to larger samples and earlier times will be of great assistance in assessing whether galaxies are the primary agents responsible for reionization. In the past year, new insight into the massive stellar populations of z > 6 galaxies has emerged from the first spectroscopic detections of UV metal emission lines. The nebular [CIII], [CIII] λλ1907, 1909 emission line has now been confidently detected in UV-selected galaxies at z = 6.03 and z = 7.73 (Stark et al. 2015a, 2016), and Stark et al. (2015b) presented the discovery of nebular CIVλ1548 emission in a z = 7.05 galaxy. Given the relatively faint fluxes of the metal lines, efforts have thus far focused on galaxies that have already been spectroscopically confirmed by Lyα. Motivated by detailed studies at lower redshift (e.g., Erb et al. 2010, Stark et al. 2014), Stark et al. (2015a) have argued that the prominent [CIII] lines reflect enhanced electron and harder radiation fields from metal-poor gas and stars, young stellar populations, and large ionization parameters. The nebular CIV emission is somewhat more challenging to reproduce, requiring an ionizing spectrum with a significant flux above 47.9 eV. The powering source of the radiation field could be a population of very hot, metal-poor, massive stars or a narrow-line AGN. Once firm constraints can be placed on other UV lines (i.e., HeII,[OIII], [CIII]), photoionization models should provide insight into whether AGN or massive stars are responsible for the energetic radiation implied by the CIV detection (e.g., Feltre et al. 2016). The presence of such intense UV metal lines is consistent with the extreme EW optical line emission suggested from the Spitzer/IRAC colors. There thus appears to be a subset of z > 6 galaxies with extreme radiation fields and moderately metal-poor gas such that collisionally excited metal lines are enhanced by the high electron of the ionized gas. Such extreme line emitters are very rare at z 2, but given the shift in the demographics of the galaxy population over 2 < z < 6, it is not surprising to see such radically different emission line spectra at z > 6. The prominent high ionization spectral features now being discovered in z > 6 galaxies are reminiscent of those in nearby metal-poor star-forming galaxies (e.g., Thuan & Izotov 2005). In Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org addition to nebular HeII and CIV emission, spectra of blue compact dwarf (BCD) galaxies have been observed with emission from [NeV] λ3426 A˚ and [FeVI]–[FeVII] (Izotov et al. 2001, 2012), requiring a source of very hard radiation with in excess of 75–100 eV. The origin of the extreme-UV photons in BCDs remains a matter of debate and may include contributions from radiative shocks, AGN activity, massive stellar populations, and X-ray binaries. The ratio of X-ray luminosity to SFR in BCDs is ten times larger than commonly found in more metal-rich galaxies (Kaaret et al. 2011), possibly suggesting an enhanced population of X-ray binaries at low metallicity. These results underscore the uncertainties that remain in the stellar and nonstellar radiation sources in metal-deficient galaxies. Efforts to better characterize and understand this local population are of great importance in the years prior to the launch of JWST. Because the UV metal lines trace the systemic redshift (Stark et al. 2014), they constrain the

typical velocity offset of Lyα (vLyα) in galaxies at z > 4. The Lyα velocity offset is governed by

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the covering fraction and kinematics of neutral hydrogen within the galaxy and plays a major role in regulating the transfer of Lyα through a partially neutral IGM. The larger the velocity offset, the less the IGM will attenuate Lyα because the line photons will be redshifted further into the

damping wing by the time they encounter neutral hydrogen. Knowledge of the typical vLyα at z > 6 is thus very important to efforts to extract constraints on reionization from the evolving Lyα transmission (see Section 5). At z 2, typical UV-selected galaxies have Lyα velocity offsets of 400–500 km s−1 (Steidel et al. 2010, Erb et al. 2014). Stark et al. (2015a) reported smaller − Lyα velocity offsets (0–120 km s 1) in a gravitationally lensed [CIII] emitting galaxy at z = 6.024, consistent with a continuation of the redshift evolution at 2 < z < 3.5 reported in Schenker et al. (2013a). Because velocity offsets are known to be smaller in galaxies with low luminosities and large EW Lyα emission (e.g., Shapley et al. 2003, Hashimoto et al. 2013, Erb et al. 2014), the trend may be explained by the shift in the galaxy population toward more prominent Lyα emission and low luminosities at higher redshift over 3 < z < 6 (see discussion in Sections 3.1 and 3.3). From a physical standpoint, the existence of small Lyα velocity offsets would arise naturally if neutral gas covering fractions are nonunity (e.g., Jones et al. 2013), because more of the Lyα radiation will escape closer to line center. Evolution in outflow kinematics could also contribute to an evolving

Lyα line profile. However it is worth noting that redshift trends in vLyα remain very uncertain owing to the small sample sizes. Recently Willott et al. (2015) have reported larger Lyα velocity − offsets (430, 504 km s 1) in two UV luminous galaxies with [CII] detections at z 6. A similarly large velocity offset is seen in a luminous [CIII] emitting galaxy at z = 7.73 (Stark et al. 2016),

suggesting that a correlation between M UV and vLyα may already be in place in the reionization era, potentially boosting transmission of Lyα through the IGM in the most luminous galaxies. We will discuss the implications of the new velocity offset measurements for efforts to use Lyα as a probe of reionization in Section 5.

4.4. Interstellar Medium Conditions and Dust Properties from the Rest-Frame Submillimeter Millimeter and submillimeter interferometers are now beginning to provide valuable constraints on the ISM gas and dust properties of star-forming galaxies in the first billion years. Thus far nearly 20 z 6.5−8 UV-selected galaxies have been observed with ALMA or the Plateau de Bure Interferometer (PdBI) (Ouchi et al. 2013, Ota et al. 2014, Capak et al. 2015, Maiolino et al. 2015, Schaerer et al. 2015, Watson et al. 2015). Continuum observations of z > 6 galaxies with ALMA and PdBI are sensitive to thermal dust emission, offering a more direct measure of the presence of dust than can be extracted from UV continuum slope observations. Spectral observations of [CII]λ158 µm and other emission lines also offer enormous potential. Given the tremendous diffi- Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org culty in securing spectroscopic redshifts in the rest-frame UV, [CII] may provide a more effective means of redshift confirmation. Moreover, the combination of [CII] with information from other rest-frame submillimeter spectral lines will ultimately enable more detailed investigations of the ISM physical conditions (density, metallicity) in individual reionization-era galaxies. Knowledge of the dust content in early galaxies is crucial to efforts to map the cosmic SFRD and offers key constraints on modes of dust production within the first billion years. The emerging body of constraints from ALMA and PdBI continuum observations demonstrates that the thermal dust emission is faint in typical star-forming z > 6 galaxies, consistent with the very blue UV continuum colors observed by HST. Continuum emission is not detected in 13 of the 18 galaxies targeted to date (Walter et al. 2012b, Ouchi et al. 2013, Ota et al. 2014, Capak et al. 2015, Maiolino et al. 2015, Schaerer et al. 2015), nor is it seen in the host galaxy of the z = 8.23 GRB (Berger et al. 2014), indicating that the rest-frame far-IR-to-UV ratios of z > 6 galaxies are smaller than

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the majority of nearby star-forming galaxies. The upper limits on the far-IR/UV luminosity ratios in the z > 6 UV-selected galaxies are instead consistent with the smaller values found in local low-metallicity dwarf or irregular galaxies (Ota et al. 2014, Maiolino et al. 2015). The low far- IR/UV luminosity ratios indicate that a smaller fraction of the UV luminosity is being absorbed and reprocessed by dust, as is commonly seen in metal-poor galaxies in the local universe. An upper limit on the total dust mass can be derived by assuming a single dust temperature (Ota et al. 2014) or a range of temperatures (Maiolino et al. 2015, Schaerer et al. 2015). These measurements 7 generally suggest dust masses that are smaller than 6–10 ×10 M (see table 1 in Watson et al. 2015). In the past year, ALMA has delivered the first dust continuum detections in the UV-selected star-forming population situated one billion years after the Big Bang. Capak et al. (2015) present detections of dust continuum in four z 5−6 UV-selected galaxies. The systems are luminous ∗ − (1–4 LUV) and have rest-frame UV colors that are somewhat redder than average at z 5 6 (β =−1.4to−0.7). The far-IR luminosities implied by the ALMA continuum detections are more than an order of magnitude lower than expected from galaxies with similar UV colors at z < 3. Capak et al. (2015) demonstrate that the galaxies detected by ALMA are consistent with the IR-excess (IRX)-β relationship found in the (SMC) but fall significantly below the Meurer et al. (1999) relationship for local starbursts. The five galaxies from Capak et al. (2015) which are not detected in the thermal continuum fall well below the SMC IRX-β relation. The emerging physical picture thus points to rapid evolution in the ISM dust properties over 2 < z < 6, implying far less obscuration of the UV luminosity, as is expected at low metallicity. Perhaps the most surprising ALMA reionization-era discovery has been the detection of ther-

mal dust emission in A1689-zD1 (Watson et al. 2015), a bright (H160 = 24.7) gravitationally lensed star-forming galaxy at z = 7.5. Unlike many of the galaxies in Capak et al. (2015), A1689-zD1 has a very blue UV slope (β =−2.0) which is typical of UV-selected galaxies at z > 7 and gives little indication of any reddening from dust. But the ALMA detection is indicative of a significant 7 −3 repository ( 4×10 M) of dust. Watson et al. (2015) infer a dust-to-gas mass ratio of 17×10 , comparable to the value in the . Based on the current body of constraints from ALMA and PdBI, A1689-zD1 does not seem to be the norm at very high redshift. Nevertheless, these ob- servations confirm that dusty, evolved galaxies are present among at least a subset of UV-selected galaxies at z > 6. As larger samples are assembled, it will become clear how common such dusty systems are in the UV-selected population of the reionization era. Over the past several years, considerable efforts have also been invested in the detection of the [CII]λ158 µm fine-structure line in UV-selected star-forming galaxies at high redshift. As the primary coolant in the ISM, [CII] is expected to be a valuable probe of the physical nature of early galaxies. Although [CII] has been easily recovered in dusty star-forming galaxies and quasar

Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org host galaxies at z 6 (Wang et al. 2013), many of the initial attempts to secure detection of [CII] in UV-selected galaxies at z > 6 have yielded nondetections (e.g., Ouchi et al. 2013, Ota et al. 2014, Maiolino et al. 2015, Schaerer et al. 2015), leading some to suggest that the local relationship between [CII] luminosity and SFR may not hold at very high redshift. However, in the past year, [CII] detections in UV-selected LBGs have begun to emerge. Riechers et al. (2014) presented [CII] detections in several z = 5.3 LBGs in the vicinity of a submillimeter galaxy, and Maiolino et al. (2015) identified [CII] emission in a z = 7.107 galaxy that is consistent with the Lyα redshift but spatially offset by 4 kpc from the centroid of the rest-UV emission. Most recently, Capak et al. (2015) reported detection of [CII] emission in nine z 5−6 galaxies. The

[CII] luminosities in the nine galaxies are fully consistent with the local L[CII]-SFR relationship, in apparent conflict with earlier studies. It is not clear why the [CII] detection rate found by Capak et al. (2015) is so much larger than in the earlier studies. In metal-poor galaxies, the harder

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radiation field may allow ionizing photons to reach larger depths in photodissociation regions, as well as in the molecular component of clouds, which may provide a significant component of the [CII] luminosity. Variations in the ionizing radiation field, metallicity, and ISM structure (patchiness and cloud surface densities) in galaxies can drive scatter in the [CII]-SFR relation (e.g., Olsen et al. 2015, Narayanan & Krumholz 2016). Indeed, a large scatter in the relation is observed in local low-metallicity galaxies with warm dust temperatures and a large filling factor of diffuse high-ionization gas (De Looze et al. 2014). This may contribute to the sample of [CII] nondetections at high z. This picture is bolstered by recent ALMA observations of the z = 7.215 LAE confirmed by Shibuya et al. (2015). Inoue et al. (2016) demonstrated that the strength of the [OIII]λ 88-µm line in this galaxy is more than 12 times that of [CII], consistent with a large filling factor of highly-ionized gas. This emerging framework should be clarified in the coming years as more robust detections emerge from ALMA. Massive dust-enshrouded galaxies (see Section 2.3) enable yet more detailed spectroscopic investigations of ISM conditions and star formation. High-resolution observations of CO and the

dust continuum have recently provided constraints on the velocity field, the CO-to-H2 conversion factor, and the relationship between star formation and gas surface density in a z = 5.66 dusty star-forming galaxy (e.g., Spilker et al. 2015). Constraints on the ISM structure are also emerging

from the growing population of dusty galaxies with [CII]andLFIR measurements (e.g., Gullberg et al. 2015). Such spectroscopic studies should become more commonplace as the number of known z 6 dusty-star forming galaxies increases.

4.5. Metallicity and Chemical Abundance Patterns The chemical enrichment of early star-forming galaxies promises to be one of the major growth areas of the coming decade. Spectroscopic observations with JWST will deliver the first direct constraints on the flux ratios of strong rest-frame optical emission lines ([OIII], [OII], Hβ,Hα, [NII], [SII]), allowing gas-phase to be inferred out to at least z 9. Prior to JWST, the spectroscopic detection of UV metal lines at z 7−8 (e.g., Stark et al. 2015a,b, 2016) provided one of the only windows on the metal content of the UV-selected galaxy population. The detection of strong [CIII] emission at z = 7.73 (Stark et al. 2016) indicates the pollution of the ISM with by 676 Myr after the Big Bang. The large EW of the UV emission lines likely reflects the enhanced electron temperatures (1.5−2 × 104 K) that are expected in low-metallicity ionized gas. Comparison of the observed UV emission line EWs to photoionization models fed by the latest version of the Bruzual & Charlot (2003) stellar population synthesis models as input (Feltre et al. 2016) suggests metallicities between 0.02 and 0.11 Z for the small sample of z > 6 galaxies with metal line detections (Stark et al. 2015a,b, 2016). Future work is required to determine how Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org representative this sample is of the early galaxy population. Absorption lines in high-resolution quasar and GRB spectra provide additional insight into the

metal content of gas in the early universe. The mass density of CIV (CIV) derived from quasar absorption line spectra decreases by a factor of two to four in the 200 Myr between z 5and z 6. The evolution is likely driven by a decrease in the metal content and ionization state of the gas surrounding early galaxies (see Becker et al. 2015 for a review). Metal absorption lines in GRB afterglow spectra also constrain the metallicity of the host galaxy ISM. Chornock et al. (2013) find that the metallicity of GRB130606A is likely in the range of [Si/H] > −1.7and [S/H] < −0.5. Element abundance ratios are also of great interest in the very metal-poor regime as they offer insight into the nature of massive stars in the earliest few generations of galaxies. Through characterization of quasar absorption line spectra at z 6, Becker et al. (2012) have placed constraints on relative metal abundances in gas reservoirs one billion years after the Big

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Bang. Based on measurements of carbon, oxygen, silicon, and iron in nine low-ionization systems at 4.7 < z < 6.3, Becker et al. (2012) find column density ratios that are similar to those seen in sub-DLAs (damped Lyα emitters) and metal-poor DLAs at lower redshifts. Given the at these high redshifts, Becker et al. (2012) argue that the elements in these absorption line systems reflect enrichment from prompt supernovae (Types II, Ib/c, and prompt Ia). The elemental abundance ratios in the nine z 5−6 systems are suggestive of nucleosynthetic yields that are fully consistent with normal Pop II stars and reveal no trace of the metal yields expected from massive Pop III stars.

4.6. Galaxy Sizes and Morphologies Measurements of galaxy sizes were first feasible at z 6 soon after the first i -band dropouts emerged (Bouwens et al. 2004a, Ferguson et al. 2004) and were extended to galaxies at z 7−8 after WFC3/IR was installed on HST (Oesch et al. 2010, Ono et al. 2012, Shibuya et al. 2015, Curtis-Lake et al. 2016). These studies demonstrate that z > 6 galaxies tend to be marginally resolved in the rest-frame UV, with average sizes in the range of 0.5 to 0.8 kpc. The light profiles suggest that star formation is centrally concentrated and not widely distributed in a clumpy large disk. Systems with disturbed morphologies make up a small fraction of UV-selected samples at z 6−8 (e.g., Oesch et al. 2010). The disturbed fraction shows no sign of evolving over 4 < z < 8 (Curtis-Lake et al. 2016). The size evolution at z > 4 has not been resolved yet. Early studies suggested that at fixed luminosity, galaxy sizes decreased slowly toward higher redshift, following a scaling close to (1 + z)−1 over 1 < z < 5 (Ferguson et al. 2004). Several recent investigations have argued that similar mild evolution continues to z 8 (Oesch et al. 2010, Shibuya et al. 2015). In contrast, Curtis-Lake et al. (2016) find no evidence that the typical sizes (defined as the mode of the log-normal size distribution) of galaxies evolve over 4 < z < 8. The tension appears to result in part from different source selection functions (see discussion in Curtis-Lake et al. 2016); as analysis techniques converge in future years, consensus on the true size evolution should emerge. An even more informative and conclusive picture will ultimately emerge once rest-optical sizes and morphologies are available from deep JWST/NIRCam imaging.

5. LYMAN-α EMITTERS AND GAMMA RAY BURSTS AS PROBES OF REIONIZATION The bulk of hydrogen reionization is thought to occur sometime in the redshift range 6 < z < 9 (e.g., Robertson et al. 2015). Because the number counts and clustering of LAEs are sensitive to the globally averaged ionization fraction of the IGM (see Dijkstra 2014 for a review), Lyα emitter

Access provided by California Institute of Technology on 01/11/17. For personal use only. > Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org galaxy surveys at z 6 offer the potential to map the progress of reionization throughout this period. Deep spectroscopy of GRBs at z > 6 provides additional constraints on the IGM through measurement of the Lyα damping wing spectra of the afterglow. Here we review the physical picture that is emerging from new constraints on reionization following dedicated spectroscopic efforts targeting LAEs and GRBs at z > 6.

5.1. Redshift Evolution of Narrowband-Selected Lyman-α Emitting Galaxies over 5.7 < z < 8.8 5.1.1. Luminosity function. The first attempt to use the observed evolution of the LAE LF to constrain reionization appeared shortly following the initial discovery of z = 6.5 LAEs (Malhotra & Rhoads 2004). The analysis revealed that the space density of LAEs does not evolve strongly

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ab10–2 0.8 M z = 5.7 UV > –20.25 Schenker et al. (2014) M UV < –20.25 Stark et al. (2011) z ] –3 = 6.6

3 0.6

– 10

Mpc z = 7.3

–1 ,25

α 0.4

= 1) 10–4 Ly

L X

log

Δ

[(

ϕ 0.2 10–5

0.0 42.0 43.0 44.0 4 5 6 7 8 log L(Lyα) (erg s–1) Redshift

Figure 6 (a) Evolution in Lyα luminosity function over redshift 5.7 < z < 7.3 as derived by Konno et al. (2014). The z = 5.7andz = 6.6Lyα luminosity functions are from Ouchi et al. (2008) and Ouchi et al. (2010), respectively. This panel is adapted with permission from Konno et al. (2014). (b) Redshift evolution in the fraction of Lyα emitting galaxies (equivalent width > 25 A˚ ), X Lyα,25,within UV-selected samples. The data points at z 7andz 8 were derived by Schenker et al. (2014), and the lower redshift points are from Stark et al. (2011). This panel is adapted with permission from Schenker et al. (2014).

between z = 5.7andz = 6.6, suggesting that the IGM is not completely neutral at z = 6.5. Since this pioneering study, the measurements of the high-redshift Lyα LFs have been revolutionized by large surveys conducted with Suprime-Cam on Subaru (see discussion in Section 2.2). The latest z = 6.5 LAE LF measurements are based on large photometric samples including as many as 200 LAEs, with contamination rates informed by extensive spectroscopic follow-up (Hu et al. 2010, Ouchi et al. 2010, Kashikawa et al. 2011). All three studies agree that the counts of LAEs decrease between z = 5.7andz = 6.5(Figure 6; also see Schechter LF parameters in Table 2), but there is some discord in the magnitude of the evolution. Hu et al. (2010) report a factor-of-two decrease in the number density of bright LAEs between z = 5.7andz = 6.5 with no change in the characteristic luminosity, whereas the Ouchi et al. (2010) and Kashikawa et al. (2011) surveys find that their LF measurements are better characterized by luminosity evolution at the 30–55% level. Deep spectroscopic integrations of faint LAE candidates in these surveys should soon be able to reconcile the three measurements. > . Access provided by California Institute of Technology on 01/11/17. For personal use only. The disappearance of narrowband-selected LAEs appears to accelerate at z 6 6. The small Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org number of LAE candidates uncovered in the Suprime-Cam surveys at z = 7.0andz = 7.3 indicates a continued downward trend in the space density of LAEs. Konno et al. (2014) report evolution in the LAE LF between 6.6 < z < 7.3at>90% confidence. Over 5.7 < z < 6.5, the integrated −5.0 density of Lyα emitting galaxies, ρLyα,isfoundtovaryas(1+ z) whereas at 6.6 < z < 7.3, the evolution requires a steeper power law, (1 + z)−20.8, suggestive of much more rapid decline in the space density of LAEs. The drop in the abundance of LAEs over 5.7 < z < 7.3 is in marked contrast to the slow evolution at redshifts 3.1 < z < 5.7. Because the UV-selected galaxy population does not show a similar accelerated decline in abundance over 5.7 < z < 7.3, the evolution in the LAE LF at z > 5.7 cannot be explained by the decline in the density of star-forming galaxies discussed in Section 3.1. Moreover, because all available evidence suggests that galaxies evolve in such a way

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to allow more Lyα to escape at earlier times (e.g., because of the decrease in the dust content and reduction in the HI covering fraction; see discussion in Section 3.2), the LAE LF evolution is not likely to arise entirely from changes in galaxy properties. The sudden drop in the visibility of LAEs at z > 5.7 is perhaps most naturally explained by evolution in the ionization state of the IGM. Quantitative constraints on the average neutral

hydrogen fraction in the IGM, xHI, can be extracted through comparison of the LF evolution to theoretical predictions of how the Lyα transmission changes as reionization progresses. The slow but significant evolution in the LAE LF at 5.7 < z < 6.5 is suggestive of a relatively low neutral hydrogen fraction throughout the IGM at z = 6.5. Kashikawa et al. (2011) estimated that the

IGM is 40% neutral (xHI 0.4) at z = 6.5 using models from McQuinn et al. (2007), and Ouchi et al. (2010) placed an upper limit on the HI fraction (xHI < 0.2) at z = 6.6 using a variety of theoretical predictions. The strong evolution in the LAE LF over 5.7 < z < 7.3 requires the IGM to have a significant neutral component at z = 7.3. Using a variety of reionization models, Konno et al. (2014) find that to match the downturn in the counts of LAEs over 5.7 < z < 7.3,

the globally averaged neutral hydrogen fraction must be in the range xHI = 0.3−0.8atz = 7.3. Constraints on the IGM ionization state are shown in Figure 7b. Although there remain systematic uncertainties in modeling the Lyα downturn (see Section 5.1.3 for more discussion), the LAE LF observations clearly indicate that the transmission of Lyα drops precipitously in the 280 Myr between 5.7 < z < 7.3, providing the first convincing evidence that the IGM evolves from mostly ionized at z 6 to somewhat neutral at z 7.3.

ab –0.5 L = 0.001 L* 1.0 ML+68% credibility interval min Robertson et al. (2013) Forced match to WMAP τ

) –1.0 0.8

–3

Mpc 0.6

–1 –1.5 Forced match

yr

☉ to W MAP τ 0.4

II

H

–2.0 Q

1– 0.2 68% credibi –2.5 interval –2 SFR density (M lity 10

10 Lyα forest transmission Dark Lyα forest pixels log 10–3 –3.0 ML SFR history Quasar near zone ML SFR history without τ constraint GRB damping wing absorption Access provided by California Institute of Technology on 01/11/17. For personal use only. Lyα emitters Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org SFR density from UV luminosity density –4 α SFR density from IR luminosity density 10 Ly galaxy clustering Lyα emission fraction –3.5 02468101214 6 8 10 12 Redshift z Redshift z

Figure 7 Contribution of galaxies to reionization as derived in Robertson et al. (2015). (a) The star formation rate density of the galaxy population over redshift. The shaded red area is the maximum likelihood star formation rate density (SFR) density history, and blue  data points are derived from integrating the UV luminosity function to 0.001 L .(b) Evolution in intergalactic medium (IGM) filling factor of neutral hydrogen (1 − QHII) predicted from SFR density history of galaxy populations. Data points show external constraints on the intergalactic medium discussed in Section 5. Both panels adapted with permission from Robertson et al. (2015). Other terms and ∗ abbreviations: GRB, gamma ray burst; L , characteristic galaxy luminosity; Lmin, minimum galaxy luminosity; Lyα, Lyman-α;ML, maximum likelihood; τ, Thompson scattering optical depth; WMAP, Wilkinson Microwave Probe.

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5.1.2. Clustering. In addition to decreasing the space density of LAEs, a partially neutral IGM will modify the observed angular distribution of LAEs on the sky. When intergalactic hydrogen is significantly neutral, LAEs will be confined to the largest intergalactic HII regions. Within these ionized gas pockets, the Lyα photons emitted by star-forming galaxies will suffer less attenuation, because the radiation will be redshifted far into the damping wing before encountering intergalactic HI. In the early stages of reionization, large ionized bubbles are very rare, produced only by the most massive overdensities in the underlying matter distribution (e.g., Barkana & Loeb 2004, Furlanetto et al. 2004). But because each bubble will contain many galaxies, the observed clustering of LAEs will be greatly enhanced when the IGM is largely neutral (e.g., Furlanetto et al. 2006, McQuinn et al. 2007, Jensen et al. 2013) In contrast to the evolution in the number counts, the correlation function of LAEs will not be as strongly impacted by changes in galaxy (i.e., dust content, HI covering fraction and kinematics). In the first half of reionization, the clustering enhancement induced by the IGM is strong enough to produce an unambiguous signal in the LAE correlation function that cannot be attributed to the intrinsic clustering of the host halos (Mesinger & Furlanetto 2008). Unfortunately, given the rapid decrease in the space density of LAEs at z > 7, it will be difficult for current facilities to deliver large enough LAE samples in the first half of reionization to reliably measure clustering. It is far more feasible to assemble large LAE samples at z = 6.5 when reionization is thought to be in its later stages. The large area narrowband surveys conducted with Suprime-Cam over the past decade have delivered the first LAE clustering measurements at z 6. The deep 1 deg2 SXDS Suprime-Cam imaging campaign has provided the best existing constraints at z = 5.7andz = 6.5. Although the IGM is expected to be highly ionized at z < 6, the clustering measurement at z = 5.7iscritical,as it provides the baseline for comparison against higher redshift measurements. Ouchi et al. (2010) reported a significant angular correlation signal in the z = 5.7andz = 6.6 LAE SXDS samples. At = . = − −1 z 5 7, Ouchi et al. (2010) derived a correlation length of r0 3 4h100 Mpc and a galaxy-dark matter bias of bg = 5.5−6.1. Future studies are required to confirm the z = 5.7 measurement, as there is some concern that these estimates may be somewhat impacted by the presence of known protoclusters in the SXDS area. The z = 6.6 LAEs in the Ouchi et al. (2010) SXDS sample have a = − −1 = correlation length of r0 2 5h100 Mpc and a bias between bg 3 and 6, providing no evidence for a sudden increase in the LAE clustering strength between z = 5.7andz = 6.6. The lack of strong enhancement in the clustering of LAEs at z = 6.6 is consistent with the reionization history implied by the LAE LF. Ouchi et al. (2010) compared the clustering measurements to theoretical predictions from McQuinn et al. (2007) and concluded that the absence of strong clustering evolution over 5.7 < z < 6.6 requires the IGM to be more than 50%

ionized at z = 6.5. Sobacchi & Mesinger (2015) confirmed this conclusion (xHI < 0.5atz = 6.6) through comparison of the Ouchi et al. (2010) angular correlation functions to an updated set of Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org reionization simulations. The larger field of view provided by the Subaru Hyper Suprime-Cam will soon provide much better measurements on the LAE angular correlation functions, significantly improving clustering constraints on the IGM ionization state.

5.2. The Lyman-α Emitter Fraction in z > 6 Lyman Break Galaxies The influx of z 6−9 LBGs that emerged following the installation WFC3/IR camera on Hubble enabled a new method of characterizing the attenuation of Lyα emission in the reionization era. As the IGM becomes increasingly neutral at z > 6, the fraction of photometrically selected

LBGs with Lyα emission above a fixed EW (X Lyα) should decrease. Unlike the narrowband LAE method discussed above, the “Lyα fraction” technique is based on spectroscopic follow-up of LBGs identified as dropouts in deep HST or ground-based imaging surveys. To first order,

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the Lyα emitter fraction, X Lyα (hereafter, the terms are used interchangeably), is determined by dividing the number of galaxies discovered with large Lyα EW by the total number that were observed spectroscopically, taking care to account for incompleteness due to photometric redshift uncertainty and obscuration by the forest of atmospheric emission lines that dominate near-IR spectra taken with ground-based facilities. A robust measure of the Lyα fraction at z 5−6 (when the IGM is known to be highly ionized) is required to provide a reference point for comparison to emerging measurements at z 7−8. Utilizing more than 600 LBG spectra at 3.5 < z < 6, Stark et al. (2010) and Stark et al. (2011) presented measurements of the Lyα fraction at 3.5 < z < 6. The analysis showed that the Lyα emitter fraction is strongly dependent on UV luminosity at a given redshift (increasing

toward lower-luminosity galaxies), indicating that redshift-dependent trends in X Lyα should be considered at fixed UV luminosity. The Lyα emitter fraction was also found to be larger among z 6 galaxies than in similarly luminous systems at z 3−4, reaching upward of 50% with rest- frame EW > 25 A˚ at z 6 (Stark et al. 2011, Curtis-Lake et al. 2012). These results again point to the Lyα transmission or production rate becoming greater toward earlier times in the redshift range 3 < z < 6, consistent with the narrowband LAE evolution discussed in Section 3.2. The changes in Lyα transmission likely arise from a reduction in dust content (indicated by UV continuum slope measurements; Section 4.1) and a smaller neutral hydrogen covering fraction (indicated by the strength of low ionization absorption lines; Section 4.3) in earlier galaxies. Following the release of the first robust samples of z 7 galaxies, it quickly became clear that the Lyα emitter fraction is much lower at z 7 (Fontana et al. 2010, Pentericci et al. 2011, Ono et al. 2012, Schenker et al. 2012). These results have been convincingly verified by follow- up surveys over the past five years. To date, well over 100 z > 6.5 galaxies have been observed spectroscopically, but only ten galaxies have been confirmed with Lyα emission at z > 7. Analyzing a sample of 102 z > 6.5 galaxies with deep spectroscopy, Schenker et al. (2014) estimated a

z 7Lyα fraction of 24% among low-luminosity galaxies (M UV > −20.25) and 8% among more-luminous (M UV < −20.25) systems (Figure 6). These results suggest that the average Lyα transmission in z 7 galaxies is 2× lower than at z 6. Similar analyses suggest that drop in Lyα transmission is even larger (3–5× when extended to z 8; Treu et al. 2013, Schenker et al. 2014, Tilvi et al. 2014). The observed drop in the Lyα fraction at z > 6 is identical to the LAE LF evolution described in Section 5.1, and is consistent with a partially neutral IGM at z 7−8. Theoretical studies initially suggested that neutral hydrogen must fill more than 50% of the IGM at z 7 to explain the drop in the Lyα transmission over 6 < z < 7 (e.g., Dijkstra et al. 2011, Pentericci et al. 2011, Schenker et al. 2012, Jensen et al. 2013, Tilvi et al. 2014). However Mesinger et al. (2015) point out that owing to the limited sample sizes, current z 7 observations remain consistent with a

Access provided by California Institute of Technology on 01/11/17. For personal use only. σ Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org highly ionized IGM at the 2 level. Sample variance from the patchy nature of reionization could

also bias X Lyα measurements (Taylor & Lidz 2014). These issues will be mitigated in the future as larger number of galaxies are observed over a wider area on the sky. Additional systematics

in the mapping between X Lyα and xHI may also be present (see extended discussion in Dijkstra 2014). An important example is the Lyα velocity offset, a crucial ingredient in determining the transmission of Lyα through the IGM. The larger the velocity offset of Lyα from systemic, the less attenuation the IGM will provide. Motivated by observations hinting at evolution in the Lyα velocity offset (Section 4.3; Stark et al. 2015a), Choudhury et al. (2015) demonstrated that existing

X Lyα observations can be fit with lower neutral hydrogen fractions (xHI 0.3atz 7andxHI 0.5 at z 8) with an evolving velocity offset. These values are notably consistent with the ionization state suggested by the LAE LF evolution described in Section 5.1. The evolution toward smaller

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Lyα velocity offsets at earlier times is consistent with the underlying shift in the demographics of the galaxy population (e.g., Stark et al. 2016). The 11 known Lyα emitters at z > 7 are thought to be galaxies that are located in the largest ionized regions in the IGM, allowing Lyα to redshift far into the damping wing before encountering intergalactic hydrogen. Recent work has begun to suggest a slightly more complex picture. The detection of nebular CIV emission in one of the 11 Lyα emitters led Stark et al. (2015b) to suggest that the visibility of Lyα in the reionization era may depend on the galaxy radiation field. Systems with hard ionizing spectra may be especially effective at ionizing the surrounding distribution of hydrogen, boosting the transmission of Lyα. In this case, the disappearance of Lyα emission at z > 6 would also depend on the frequency of extreme radiation fields in reionization- era galaxies, adding uncertainty to models of the evolving transmission of Lyα. Perhaps equally puzzling is the detection of strong Lyα emission in each of the four luminous z > 7 galaxies identified as UV-dropouts in Roberts-Borsani et al. (2016). The galaxies stand out as being among the brightest galaxies known at these redshifts and each has IRAC colors pointing to large EW optical emission lines (see discussion in Section 4.2). How Lyα is able to escape from these systems while being extinguished in most other z > 7 galaxies is not known. Zitrin et al. (2015) suggest that the preselection of galaxies with red IRAC colors may identify galaxies with hard radiation fields capable of enhancing Lyα transmission, similar to the CIV emitter identified in Stark et al. (2015b). As these are among the most luminous galaxies known at z 7−9, it is also conceivable that they are situated in overdensities that have created very large ionized bubbles (e.g., Barkana & Loeb 2004, Furlanetto et al. 2004), thereby increasing the transmission of Lyα. Given that IGM attenuation is lowest in galaxies with large velocity offsets, Stark et al. (2016) argue that the tendency for Lyα velocity offsets to be largest in luminous galaxies (Erb et al. 2014) may also play a role. Measurement of the systemic redshift (via detection of [CIII] emission) in one of

the four galaxies confirms a large value of vLyα (see Section 4.3 and Stark et al. 2016), offering tentative support for this hypothesis and suggesting that Lyα may be most readily detectable in rare luminous galaxies found in the widest area surveys. Regardless of the precise explanation, it is clear that spectroscopy is beginning to provide the first steps toward a much more sophisticated picture of how Lyα is transmitted through a partially neutral IGM.

5.3. Gamma Ray Burst Afterglows GRB afterglows provide a sensitive measure of the transparency of the IGM to Lyα. GRBs may provide a less biased probe of the IGM than quasars, as they are expected to trace rather typical sites of massive star formation in contrast to the massive dark matter halos likely traced by luminous

Access provided by California Institute of Technology on 01/11/17. For personal use only. > . = . Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org quasars. Of the five known GRBs at z 5 9, only GRB 130606A (z 5 913) has a spectrum of comparable S/N to high redshift quasars. The spectrum of GRB 130606A shows a cutoff blueward of 8410 A˚ owing to Lyα absorption. The transmission fraction in the Lyα forest is consistent with what is seen in quasars at similar redshifts. Chornock et al. (2013) place a 2σ upper limit on the neutral hydrogen fraction of <0.11, based on the lack of a red Lyα damping wing. Spectral observations of GRB 140515A (z = 6.33) also provide useful constraints. Chornock et al. (2014) show that the red damping wing can be reproduced with a constant IGM neutral hydrogen = . +0.011 . < < . fraction xHI 0 056−0.027 over the redshift range 6 0 z 6 3. Alternatively, a hybrid model with a host galaxy absorbing system and partially neutral IGM (xHI = 0.12 ± 0.05) can fit the data equally well. Although clearly some degeneracy exists in the interpretation, the spectrum of

GRB 140515A strongly suggests that the IGM is not significantly neutral at z 6−6.3(xHI < 0.21 at 2σ ). Damping wing analyses of the afterglow spectra of GRB 050904 (z = 6.295) and GRB

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080913 (z = 6.733) have placed upper limits on the neutral hydrogen fraction of xHI < 0.6at z = 6.295 (Totani et al. 2006) and xHI < 0.73 at z = 6.733 (Patel et al. 2010). The Lyα and GRB studies thus present a consistent picture in which the IGM transitions from

mostly ionized at z 6−6.5(xHI < 0.2) to considerably neutral (xHI 0.3−0.8) at z 7−8. The neutral hydrogen fractions implied by these studies are shown in Figure 7b. In the following sec- tion, we will investigate whether the ionizing output expected from star formation in galaxies is ca-

pable of ionizing the IGM in a timescale that is consistent with the xHI constraints discussed above.

6. EARLY GALAXY GROWTH AND CONTRIBUTION TO REIONIZATION 6.1. Cosmic Evolution of Star Formation Rate Density and Stellar Mass Density The improved census of galaxies at z > 6 over the past decade has allowed the first robust constraints to be placed on the history of star formation and stellar mass growth in the first billion years (see Madau & Dickinson 2014 for a detailed review). Calculation of the SFRD from the UV LF requires a correction for dust and a transformation between the UV continuum luminosity and the SFR. The UV LF is typically corrected for dust extinction using the Meurer

et al. (1999) relationship between extinction and UV slope (AUV = 4.43+1.99β). Using the latest β-M UV relationships (Section 4.1), Bouwens et al. (2015b) derived mean dust extinctions (in units

of LIR/LUV+1) of 2.4, 2.2, 1.8, 1.66, 1.4, and 1.4 for dropout samples at z 4, 5, 6, 7, 8, and 10. The dust-corrected UV luminosity can then be related to the star formation rate via a conversion

factor (defined here as κ1500) that is sensitive to the stellar population properties (age, star formation

history, metallicity): SFR = L1500 × κ1500. The conversion factor is typically derived from stellar population synthesis models (e.g., Bruzual & Charlot 2003). Most SFR measurements at z > 4 −28 −1 −1 assume κ1500 = 1.25×10 M yr erg s Hz, a value that is adapted from Kennicutt (1998) and assumes a Salpeter with mass limits of 0.1 and 100 M and continuous star formation in excess of 100 Myr. For smaller star formation timescales (as may be indicated by the large sSFRs at z > 5), a stellar population will produce less UV luminosity for a given SFR, leading to slightly larger conversion factors. In the following, we will assume the canonical conversion factor quoted above, but we note that SFRs could be underestimated by a factor of up to 2 if stellar populations have been undergoing continuous star formation for less than 100 Myr. Variations in stellar metallicity can lead to a similar level of uncertainty in the conversion factor (Madau & Dickinson 2014). These systematic uncertainties in SFRs will decrease as knowledge of the IRX-β relation (see discussion in Section 4.4) and stellar populations improves following future observations in the far-IR and rest-UV and optical with ALMA and JWST. Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org When computing the SFRD, a lower-luminosity limit must be chosen when integrating the

luminosity function. At z > 4, it has become customary to use M UV =−17.7 (which is equivalent −1 to SFR = 0.7 M yr ), corresponding to the magnitude limits of current surveys at z 7−8. Between z 8andz 4, the dust-corrected SFRD increases by 12–14× when integrated to this limit (e.g., Bouwens et al. 2015b, Finkelstein et al. 2015). Madau & Dickinson (2014) adopted a −2.9 SFRD fitting function that simplifies to ρSFR(z) ∝ (1 + z) at 3 < z < 8. The latest UV LF constraints at z > 4 give slightly steeper power laws when the SFRD fit is limited to 4 < z < 8: −4.3 Finkelstein et al. (2015) find that the SFRD ρSFR(z) ∝ (1 + z) , and the Bouwens et al. (2015b) −3.9 data are consistent with ρSFR(z) ∝ (1+z) . These results are broadly consistent with one another and suggest a picture whereby the overall density of star formation in the Universe rises smoothly in the 900 Myr between z 8andz 4. Perhaps not surprisingly given the limited numbers of galaxies known at z > 8, the form of the SFRD evolution at z > 8 is more controversial. Adopting

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the same luminosity limit, Oesch et al. (2014) find that the z > 8 SFRD derived from CANDELS −10.9 and the UDF/XDF fields is better fit by a steeper power law ρSFR(z) ∝ (1 + z) , suggesting a period of accelerated evolution over 8 < z < 10. Other studies (Ishigaki et al. 2015; McLeod et al. 2015, 2016) find that the counts of z 9 galaxies in the parallel fields from the HFF survey support shallower SFRD evolution, consistent with the trend found over 4 < z < 8. It is conceivable that cosmic variance could be responsible for the discrepancies. Improved constraints on the SFRD evolution at z > 8 should emerge from completion of ongoing HST campaigns (see Section 3.2), and deep imaging with JWST will conclusively determine the census of UV-selected galaxies at z > 8 by the end of the decade. The rate of GRBs at high redshift can provide a complementary constraint on the SFRD. Intriguingly, current measurements reveal that the GRB rate does not fall off as quickly as the SFRD shown in Figure 7a. If the GRB rate is normalized to the low-redshift SFRD, it suggests a density of star formation that is three to six times larger at z > 4 than the dust-corrected UV-based estimates discussed above (e.g., Robertson & Ellis 2012, Perley et al. 2015). This has led some to suggest that the GRB efficiency may be higher within the first two billion years of , as might be expected if the GRB rate depends on host metallicity. Future progress in understanding the apparent discrepancy will require both larger GRB samples at z > 4and detailed characterization of the host properties. The SMD can be determined by integrating the stellar mass functions derived for high-redshift dropout samples. Duncan et al. (2014) have derived the SMD using stellar mass limits of 108 and 13 10 M, demonstrating that the mass density in stars increases by a factor of five between z 7 and z 4, broadly consistent with earlier measurements (e.g., Gonzalez´ et al. 2011, Lee et al. 2012, Stark et al. 2013). The stellar mass densities at z 4−7 have been shown to be consistent with the integral of the SFRD once the recycling fraction is taken into account (e.g., Madau & Dickinson 2014). Comparison of the mass density in place at the end of the reionization era reveals that less than 1% of stars in place today formed at z > 6. Oesch et al. (2014) have recently provided

the first estimate of the SMD for galaxies more luminous than M UV =−18 at z 10 (log10 ρ = +0.5 −3 4.7−0.8 M Mpc ), an order of magnitude less than the mass density at z 8. If larger samples confirm this result, it would indicate a rapid period of stellar mass growth at z > 8. As emphasized in Section 4.2, considerable uncertainty remains in the stellar masses of z > 6 galaxies, owing to nebular contamination and the mismatch in depth between Spitzer and HST imaging. Deep JWST imaging at 2–5 µm will rectify both issues, delivering vastly improved measurements of the evolving SMD to very early cosmic epochs.

6.2. Contribution to Reionization Access provided by California Institute of Technology on 01/11/17. For personal use only.

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org Star-forming galaxies have long been suggested as the primary agents responsible for reionization of intergalactic hydrogen. Progress in measuring the luminosity function and spectral properties of star-forming galaxies at z > 6 have motivated numerous investigations of the contribution of galaxies to reionization in recent years (e.g., Bolton & Haehnelt 2007; Kuhlen & Faucher-Giguere` 2012; Robertson et al. 2013, 2015; Bouwens et al. 2015a; Stanway et al. 2016). The general objective of these studies is to determine whether the ionizing output of star-forming galaxies is sufficient to achieve reionization by z 6 (as required by z > 5 quasar absorption line spectroscopy; e.g., McGreer et al. 2015) while also supplying the IGM with enough free to reproduce the latest measurements of the optical depth to Thomson scattering faced by CMB photons. The first step in tabulating the contribution of galaxies to reionization is to integrate the

redshift-dependent UV LF to a low-luminosity limit at z > 6. This gives an estimate of ρUV,the far UV luminosity density. The cosmic ionization rate produced by star-forming galaxies can then

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be calculated from the UV luminosity density,

nion = fesc ξion ρUV, (9)

where fesc is the fraction of hydrogen ionizing photons that escape galaxies and ξion is the number of hydrogen ionizing photons produced per second divided by the emergent far UV luminosity at 1500 A˚ . The importance of the escape fraction to the picture of reionization has motivated many attempts to characterize the output of LyC photons from star-forming galaxies. These experiments must be performed at z < 4 as the IGM is optically thick to LyC photons at higher redshifts. The search for LyC emission from z 3−3.5 galaxies has produced many promising candidates, but only three robust LyC emitters have been identified (Vanzella et al. 2012, 2015; Mostardi et al. 2015). Although the absolute escape fraction in LyC emitting galaxies is estimated

to be in the range fesc = 14−19% (Mostardi et al. 2015), such objects appear very rare among UV luminous galaxies at z 3−4. There have been some reports that LyC detections are more common in less-luminous galaxies (e.g., Nestor et al. 2013), but further work is required to confirm the foreground contamination rate in these fainter systems. Estimates of the LyC production

efficiency, ξion, require constraints on the shape of the ionizing spectrum. Using knowledge of the stellar populations inferred from UV continuum slope constraints, it is possible to place estimates

on ξion using stellar population synthesis models. Using the UV slopes from Dunlop et al. (2013) and Bruzual & Charlot (2003) stellar population models, Robertson et al. (2013) derived a value −1 of log {ξion/[Hz erg ]}=25.2, comparable to estimates used in other reionization studies (e.g., Kuhlen & Faucher-Giguere` 2012). The progress of reionization is calculated through comparison of the cosmic ionization rate and IGM recombination rate. The IGM ionization state is commonly denoted by the parameter

QHII, the filling factor of ionized hydrogen in the IGM. The time evolution of QHII is determined by a differential equation comparing the cosmic ionization and recombination rates as a function of redshift at z > 6,

˙ = n˙ ion − QHII , QHII (10) nH trec

where nH is comoving hydrogen density and trec is the IGM recombination timescale. Following Kuhlen & Faucher-Giguere` (2012), the recombination time can be approximated as

 −  − .  − 1 + z 3 T 0 7 C 1 t = 0.88 Gyr 0 HII , (11) rec 7 2 × 104 K 3

where T 0 is the temperature of the ionized hydrogen gas and CHII is the clumping factor 2 2 = Access provided by California Institute of Technology on 01/11/17. For personal use only. ( n / nH ) of ionized hydrogen. Simulations suggest that a clumping factor of CHII 3is

Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org H appropriate for reionization calculations (e.g., Pawlik et al. 2009). Once the redshift-dependent

cosmic ionization and recombination rates have been tabulated, the evolution of QHII can easily be tabulated by solving the differential equation. The final step is to determine whether the cosmic ionization rate provided star-forming galaxies is sufficient to reproduce the optical depth to electron scattering faced by the CMB. The Thomson

scattering optical depth can be calculated from QHII as follows:  z −1 2 τ(z) = c nH σT fe QHII(z )H (z )(1 + z ) dz , (12) 0

where c is the , fe is the number of free electrons per hydrogen nucleus, and σT is the Thomson scattering cross section.

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The most recent calculations suggest two requirements must be met for galaxies to fully reion- ize the IGM by z 6. First, there must be an abundant population of faint star-forming galaxies below the sensitivity limit of current z > 6 surveys. The minimum galaxy luminosity required to achieve reionization varies somewhat from study to study, but typical values are in the range

M UV,min =−10 to −13 (e.g., Kuhlen & Faucher-Giguere` 2012, Robertson et al. 2013, Bouwens et al. 2015a), corresponding to 4–7 magnitudes below the current detection limits of HST blank- field imaging campaigns. The only way of testing this requirement using current facilities is through characterization of faint gravitationally lensed galaxies in deep imaging of massive lens- ing clusters. The latest results from HFF suggest that the counts of galaxies continue to rise to

M UV =−15 (Atek et al. 2015a, Livermore et al. 2016), consistent with deeper surveys con- ducted at z 2 (Alavi et al. 2014). However, lensed samples are very small and remains significant at these very low luminosities (Robertson et al. 2015). Ultimately, ultradeep infrared imaging with JWST will be required to provide robust constraints on the shape of the

UV luminosity function at luminosities below M UV =−17. The other condition is that the galaxy population must be fairly efficient ionizing agents in the

reionization era, requiring large ionizing photon escape fractions ( fesc = 10−20%), well above the typical value found in luminous LBGs at z 3. Indications that the escape fraction is larger in low-luminosity galaxies at z 3 (Nestor et al. 2013) and may increase with redshift at z > 3(e.g., Jones et al. 2013; see Section 4.3) suggest that such large escape fractions may not be unreasonable. Conroy & Kratter (2012) have argued that runaway massive stars, a relatively common population in the Milky Way, may enhance the escape fraction of ionizing radiation in early galaxies. The high speeds of runaway stars allow them to travel distances 0.1–1 kpc during their lifetimes, comparable to the sizes of z > 6 galaxies. The ionizing flux from stars positioned at such large radii will be

much more likely to escape the galaxy. The fesc requirement could alternatively be eased somewhat if the ionizing radiation field is more extreme than indicated by UV continuum slopes. Recent studies have demonstrated that the addition of binary to population synthesis models boosts the hydrogen ionizing flux by up to 60% for low metallicity galaxies (Stanway et al. 2016). Empirical evidence for hard ionizing spectra in reionization-era galaxies has emerged following the detection of nebular CIV emission in a star-forming system at z = 7.05. Using a suite of photoionization models described in detail in Feltre et al. (2016), Stark et al. (2015b) estimated −1 a LyC photon production efficiency value of log (ξion/Hz erg ) = 25.68, larger than that used in most reionization calculations. More recently, Bouwens et al. (2015d) calculated similar values −1 (log {ξion/Hz erg } = 25.28–25.34) for galaxies with blue UV slopes, using Hα EWs derived from Spitzer flux excesses in galaxies at 3.8 < z < 5.0 (see Section 4.2). Independent evidence that galaxies become more efficient ionizing agents at earlier times is provided by Becker & Bolton (2013) based on a comparison of the UV ionizing background inferred from the Lyα forest and

Access provided by California Institute of Technology on 01/11/17. For personal use only. > Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org the cosmic ionization rate implied by the galaxy population. Spectroscopic investigations of z 6

galaxies will be revolutionized by JWST, promising much improved constraints on ξion in the near future. Direct constraints on fesc at z > 6 will be more challenging, but it is possible that z > 7 galaxies with significant LyC leakage will be identifiable with JWST through comparison of Hβ and UV continuum SFRs (e.g., Zackrisson et al. 2013). If galaxies are the primary ionizing agents through the reionization era, their ionizing output may also be sufficient to explain the optical depth to electron scattering faced by CMB photons. But the Thomson scattering optical depth derived by WMAP (τ = 0.089 ± 0.014; Bennett et al. 2013) was shown to require a significant population of star-forming galaxies at z 10−15, in striking conflict with the rapidly declining abundance of galaxies at z > 8 (e.g., Robertson et al. 2013). The tension has been greatly reduced following the lower optical depth (τ = 0.058 ± 0.012) reported by the Planck Collaboration (2016). Recent studies have shown that it is now possible to

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match the electron scattering optical depth without requiring a large contribution from galaxies at z > 10 (Bouwens et al. 2015a, Robertson et al. 2015). Current models that reproduce the Thomson optical depth predict that the bulk of reionization occurs at 6 < z < 9 and the IGM is close to 50% neutral by z 7, a significant departure from earlier studies which required reionization to be significantly under way at z 9−10.

SUMMARY POINTS 1. Deep imaging surveys with HST/WFC3 have provided the first large census of star- forming galaxies within the first billion years of cosmic time. The period between z 8 and z 4 witnesses a 12–14-fold increase in the global SFRD. The luminosity func- tion in the reionization era is characterized by a very steep faint-end slope, reaching α −2byz 7−8, indicating that low-luminosity galaxies dominate the integrated UV luminosity density throughout the reionization era. Existing samples at z 9−10 hint at the possibility of an accelerated decline in the SFRD. 2. The physical properties of galaxies have been characterized through a wide range of multiwavelength observations. The emerging picture suggests that early star-forming galaxies are compact, low stellar mass systems with relatively little dust obscuration and large sSFRs. New spectroscopic constraints reveal emission lines that require moderately metal-poor gas and a harder radiation field than is seen in galaxies at lower redshift. Absorption lines tracing low-ionization metals appear to grow weaker at earlier times. This points to a low covering fraction of neutral gas, potentially allowing for a larger escape fraction of hydrogen-ionizing photons. 3. The transmission of Lyα through galaxies increases among higher redshift systems over 3 < z < 6, likely owing to the reduced dust content and lower covering fraction of HI. The visibility of LAEs falls off rapidly at still earlier times, suggesting a picture

whereby intergalactic hydrogen transitions from mostly ionized at z 6−6.5(xHI < 0.2) to considerably neutral at z 7−8(xHI 0.3−0.8). 4. The z > 6 ionizing budget suggested by new measurements of the UV LF and early galaxy properties is sufficient to complete reionization by z 6 and reproduce the Thomson scattering optical depth measured by Planck Collaboration (2016), provided that the

luminosity function extends to M UV −13 and that a large enough fraction of ionizing

radiation escapes galaxies ( fesc 0.2). Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org FUTURE ISSUES 1. Knowledge of the census of star formation and stellar mass in the early universe will again be revolutionized following the launch of JWST in 2018. NIRCam imaging will robustly extend measurements to z 10−15, quickly resolving whether or not there is an accelerated decline in the SFRD at z > 8. The improved 3–5 µm photometry provided by NIRcam will allow substantial improvements in the determination of the SMF and sSFR evolution. Ultradeep NIRCam data will probe whether the UV LF continues to

rise to M UV −13, as is required for galaxies to achieve reionization.

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2. The metal content and relative elemental abundances in early galaxies are a frontier that promises to take off in the coming years. JWST/NIRSpec will deliver rest-optical emission line spectroscopy for galaxies at z 6−11, providing constraints on oxygen abundance using standard strong line calibrations. Relative chemical abundances from galaxies, quasar absorption lines, and GRB afterglow spectra will offer insight into the nucleosynthetic yields of early populations of massive stars. 3. The nature of massive stars in early galaxies is another emerging topic of interest. Re- cent results from ground-based facilities suggest that hard radiation fields are present in z 7−8 galaxies, but samples are limited and may be biased. The combination of JWST/NIRSpec and future ground-based 20–40-meter class telescopes will provide the requisite constraints necessary to characterize the ionizing spectrum and the nature of the massive stars present in early galaxies. 4. One of the most recent developments in the study of z > 6 galaxies has come from ALMA. The physical picture emerging from these data is still very much in flux. In the coming years, ALMA will lead to major improvements in our understanding of the dust content of early galaxies; future surveys promise to to characterize the physical conditions in the ISM at high redshift. Continued efforts with submillimeter and millimeter facilities are required to constrain the contribution of z > 6 dusty star-forming galaxies to the cosmic SFRD. 5. Future LAE surveys with Hyper Suprime-Cam and the Prime Focus Spectrograph on Subaru will greatly improve measurements of the decline in LAE visibility over 6 < z < 7.3. New constraints on the Lyα LF and clustering from these campaigns will lead to considerable progress in constraints on the IGM ionization state.

DISCLOSURE STATEMENT The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

ACKNOWLEDGMENTS I thank Edo Berger, Rychard Bouwens, Stephane´ Charlot, Richard Ellis, Wen-Fai Fong, Ian McGreer, Dan Marrone, Desika Narayanan, Brant Robertson, Justin Spilker, and Brian Siana for Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org useful discussions that helped formulate this review and am grateful for the hospitality of Le Centre de Recherche Astrophysique de Lyon, where much of this review was written. I acknowledge support from the National Science Foundation through grant AST-1410155.

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Annual Review of Astronomy and Astrophysics

Volume 54, 2016 Contents

A Fortunate Half-Century Jeremiah P. Ostriker ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp1 The Remnant of Supernova 1987A Richard McCray and Claes Fransson pppppppppppppppppppppppppppppppppppppppppppppppppppppppp19 Astrophysics with Extraterrestrial Materials Larry R. Nittler and Fred Ciesla pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp53 Red Clump Stars L´eo Girardi pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp95 onto Pre-Main-Sequence Stars Lee Hartmann, Gregory Herczeg, and Nuria Calvet ppppppppppppppppppppppppppppppppppppp135 Interstellar Hydrides Maryvonne Gerin, David A. Neufeld, and Javier R. Goicoechea ppppppppppppppppppppppppp181 The Quest for B Modes from Inflationary Gravitational Waves and Ely D. Kovetz ppppppppppppppppppppppppppppppppppppppppppppppppppp227 Gravitational Instabilities in Circumstellar Disks Kaitlin Kratter and Giuseppe Lodato ppppppppppppppppppppppppppppppppppppppppppppppppppppppp271 The Evolution of the Intergalactic Medium Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org Matthew McQuinn pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp313 The Magellanic Stream: Circumnavigating the Galaxy Elena D’Onghia and Andrew J. Fox ppppppppppppppppppppppppppppppppppppppppppppppppppppppp363 Masses, Radii, and the Equation of State of Stars Feryal Ozel¨ and Paulo Freire ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp401 The Eccentric Kozai-Lidov Effect and Its Applications Smadar Naoz pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp441 Protostellar Outflows John Bally ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp491

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The Galaxy in Context: Structural, Kinematic, and Integrated Properties Joss Bland-Hawthorn and Ortwin Gerhard pppppppppppppppppppppppppppppppppppppppppppppp529 Structure and Kinematics of Early-Type Galaxies from Integral Field Spectroscopy Michele Cappellari pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp597 Six Decades of Spiral Density Wave Frank H. Shu ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp667 Gamma-Ray Observations of Active Galactic Nuclei Grzegorz (Greg) Madejski and Marek Sikora pppppppppppppppppppppppppppppppppppppppppppp725 Galaxies in the First Billion Years After the Big Bang Daniel P. Stark pppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp761

Indexes

Cumulative Index of Contributing Authors, Volumes 43–54 ppppppppppppppppppppppppppp805 Cumulative Index of Article Titles, Volumes 43–54 ppppppppppppppppppppppppppppppppppppp808

Errata

An online log of corrections to Annual Review of Astronomy and Astrophysics articles may be found at http://www.annualreviews.org/errata/astro Access provided by California Institute of Technology on 01/11/17. For personal use only. Annu. Rev. Astron. Astrophys. 2016.54:761-803. Downloaded from www.annualreviews.org

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