Red Supergiants As Extragalactic Abundance Probes: Establishing the J-Band Technique

Red Supergiants as Extragalactic Abundance Probes: Establishing the J-Band Technique

J. Zachary Gazak

Thesis Committee: Rolf Kudritzki (Chair), Josh Barnes, Fabio Bresolin, Ben Davies, Lisa Kewley, John Learned, and John Rayner

ABSTRACT

We propose to study the metallicity evolution of star forming galaxies and the expanding universe by developing, calibrating, and utilizing methods to extract elemental abundances from quantitative spectroscopy of red supergiants (RSGs). The extreme IR luminosities of RSGs allow for spectroscopic observations over extragalactic distances. By observing a population of RSGs in a target galaxy, the current enrichment as a function of spatial position allows insight into the evolution of the galaxy. By bypass- ing current methodologies (which demand spectral resolutions in excess of R=20,000) in favor of newly proposed analysis techniques requiring more modest resolutions of R ∼3000 in the J band (1.15 - 1.23 µm), our observational efficiency will far exceed current standards. With multi-object capable instruments on both Keck and Subaru we can extend what is possible both in terms of objects observed and accessible distances. In recent years, the advent of quantitative spectroscopy of extragalactic blue supergiants has revolutionized how we understand chemical enrichment and extragalactic distances while exposing significant drawbacks in the assumptions and calibrations of non-stellar techniques. By extending our knowledge to an additional population we conduct a critical test of the existing techniques, increased confidence and spatial resolution in metallicity gradients, and expose a rich new source of information on α/Fe abundances. Furthermore, we pose ourselves to fully exploit the capabilities of future telescopes. Current instruments on Keck and Subaru allow for studies up to 10 Mpc while the next generation of extremely large telescopes will extend this range to at least 30 Mpc, at which point the techniques can be unleashed on entire galaxy clusters. Thus the investment of effort and telescope time for this project are justified by immediate significant scientific returns and the promise that those returns will be dwarfed by future applications of the techniques developed. 1. Introduction

Measuring the chemical composition of−and distances to−star forming galaxies have become pivotal goals for modern astrophysics due to the insight they provide into the evolution of galaxies and of the expanding universe. Unraveling the chemical enrichment history of the universe dictates first a clear understanding of the stars which drive that enrichment and the galaxies providing for the formation of those stars. While a solid understanding of the formation and evolution of galaxies remains elusive, the relationship between central metallicity and galactic mass appears to be a critical component (Lequeux et al. 1979; Tremonti et al. 2004; Maiolino et al. 2008), and the metallicity gradient provides a wealth of information needed to describe the complex dynamics of galaxy evolution, including clustering, merging, infall, galactic winds, star formation history, and IMF (Prantzos & Boissier 2000; Colavitti et al. 2008; Yin et al. 2009; Sánchez-Blázquezet al. 2009; De Lucia et al. 2004; de Rossi et al. 2007; Finlator & Davé2008; Brooks et al. 2007; Köppen et al. 2007; Wiersma et al. 2009).

However, as intriguing as the observations of the mass-metallicity relationship and the metallicity gradients of galaxies are, the published results are highly uncertain. They rely on spectroscopy of H ii region emission lines, mostly oxygen, and the “strong line” analysis method, which uses the fluxes of the strongest forbidden lines (most often [OII] and [OIII]) relative to Hβ (Kewley & Ellison 2008). These methods depend strongly on the choice of calibration, and utilizing different commonly accepted calibrations yields varying and sometimes conflicting results from the same data sets (Fig. 1). Such results have undermined the confidence in observations based on these methods (Kewley & Ellison 2008; Kudritzki et al. 2008; Bresolin et al. 2009). Furthermore, the strong line (collisional) metallicities tend to disagree with recombination line metallicities from the same H ii regions, which measure 0.2 to 0.3 dex higher. Finally, above roughly half solar metallicity, spectral line saturation effects prevent accurate abundance measurements (Stasińska 2005). As we reach to higher and higher redshifts with galaxy surveys containing incredible numbers of sources, new observations and calibrations are needed on the most nearby galaxies. The ideal targets for such work are the drivers of galaxy evolution and enrichment−the young, high mass stars which convert hydrogen and helium into heaver elements and deliver those nuclear processed materials back into the interstellar medium (ISM).

Indeed it is these massive stars which represent the most salient tracers of the metallicity structure of galaxies as their spectra are imprinted with atmospheric chemical compositions and their short lifetimes dictate that this composition mirrors that of their local ISM. Because of this direct link to galaxy evolution, the impact of investigations targeting individual stars in galaxies has become a convincing motivator for investments of time on the largest telescopes in the world. This new focus is due in part to the advent of atmospheric spectral synthesis modeling, through which spectra of individual stars can be quantitatively studied to extract basic stellar parameters (temperatures and gravities) and accurate chemical abundances of many elements. In this way only the physics of radiative transfer and understanding of atomic structure are needed: few or no empirical calibration techniques are required. Another key development has been the multi-object spectrograph (MOS) which provides an eﬃciency boost by allowing for the simultaneous collection of many spectra. MOS based instruments make extragalactic stellar astronomy possible even when required integration times reach and sometimes exceed a full night per target.

2 (a) (b)

Fig. 1.— (a) The mass-metallicity relationship of star forming galaxies in the nearby universe obtained by applying several widely used empirical metallicity calibrations based on different strong line ratios. This figure illustrates that there is an effect not only on the absolute scale, but also on the relative shape of this relationship. Adapted from Kewley & Ellison (2008). (b) H ii region galactocentric oxygen abundance gradients in the spiral galaxy NGC 300 obtained from the same dataset but different strong line calibrations: McGaugh 1991 = M91, Tremonti et al. 2004 = T04, Kewley & Dopita 2002 = KD02, and Pettini & Pagel 2004 = PP04, as shown by the labels to the corresponding least squares fits (from Bresolin et al. 2009). These abundances are compared with auroral line-based abundances determined by Bresolin et al. (2009), which are shown by the full and open circle symbols, and the corresponding linear fit is shown by the continuous line. R25 is the isophotal radius (5.33 kpc). See also Fig. 2.

To date the majority of extragalactic quantitative stellar spectroscopy has been undertaken using blue supergiants (BSGs) with observations at optical wavelengths (Bresolin et al. 2001, 2002). BSG chemical compositions are extracted using quantitative techniques with low resolution spectra (Urbaneja et al. 2003; Kudritzki et al. 2008). These techniques have been applied to a number of Local Group galaxies (WLM − Bresolin et al. 2006; Urbaneja et al. 2008; NGC 3109 − Evans et al. 2007; IC1613 − Bresolin et al. 2007; M33 − U et al. 2009), with continued eﬀorts by our group both nearby (M33, NGC6822) and at greater distances (M81, NGC2403).

The intent of this dissertation is twofold. First, we plan to develop and exploit techniques for a new, independent stellar population capable of providing reliable chemical enrichment information and an additional test on the BSG methods discussed above. Red supergiants (RSGs) are young, extremely luminous stars which emit strongly in the IR and represent a natural choice for this new population. Second, we will utilize the dominance of these stars on the integrated IR light of young super star clusters (SSCs) to trace metallicities of coeval populations at a larger variety of ages and to greater distances. RSGs are evolutionary successors to BSGs and as such evolve from stars of similar initial mass and thus represent with equal ﬁdelity the chemical makeup of the young stellar population. The extreme luminosities of RSGs peak at ∼ 1µm with absolute J-band magnitudes of MJ = -8 to -11. Their spectra in this bandwidth are rich with features providing diagnostics for extraction of accurate abundances of multiple elements. To date the crippling limitation of utilizing RSGs for extragalactic work has been the need for spectral resolution in

3 Fig. 2.— Radial oxygen abundance gradient obtained from H ii regions (circles) and blue supergiants (star symbols: B supergiants; open squares: A supergiants). The regression to the H ii region data is shown by the continuous line. The dashed line represents the regression to the BA supergiant star data. For reference, the oxygen abundances of the Magellanic Clouds (LMC, SMC) and the solar photosphere are marked. From Bresolin, Gieren, Kudritzki et al. (2009). excess of R=20,000. Recent work by collaborators has demonstrated a promising technique to access accurate chemical composition information from quantitative spectroscopy in the J band (1.15 - 1.23 µm) with resolution requirements of only R ∼3000 (Davies et al. 2010). As a result of this and instrumentation which is currently coming online at Keck (MOSFIRE), we have the opportunity to exploit RSGs as an independent probe of chemical enrichment to distances of up to ∼10 Mpc. While this work is posed to provide crucial results, the future implications are even brigher. With the next generation of ELTs−equipped with multi-conjugate adaptive optics optimized in the IR−this range can be extended to at least 30 Mpc and possibly out to ∼100 Mpc, meaning that the RSG populations of entire clusters of galaxies will become accessible targets (Evans, Davies, Kudritzki et al., 2010).

The J-band offers significant benefits over other IR windows. First, molecular absorption lines of OH, H2O and CO are weak and appear as a pseudo-continuum at low resolution. This leaves as dominant spectral features Fe and the α-elements Si, Ti, and Mg−leaving RSGs unique in that they provide a direct measurement of iron and alpha element abundances. In general, these features are separated such that blending is not an issue and of sufficient strength to study at low resolution. And the IR excess due to circumstellar material often found in RSGs contributes only negligibly to the J-band, such that there is no dilution of the spectral features. Second, the J-band is well-covered by existing and planned instrumentation as it is an ideal wavelength range to take full advantage of adaptive optics performance. Finally, interstellar reddening is suppressed in comparison to visible wavelengths which allows more freedom in target selection and simplifies analysis techniques.

This project will commence in three stages and, with collaborators with connections to ESO will cover both hemispheres of the sky. First, we will conduct rigorous tests of the technique and calibrate it to established high resolution H and K band spectroscopy of RSGs and a coeval population of BSGs by conducting observations of RSGs in the Perseus OB-1 association. To this end we will propose for one night on Subaru (IRCS) and one night on IRTF (SpeX) during the

4 2011B semester to build a sample of high and low resolution spectra. In collaboration with Miguel Urbaneja, who has already obtained spectra of all known BSGs in Per OB-1, we will conduct a thorough investigation utilizing multiple stellar populations and establishing for the ﬁrst time any necessary calibrations between the techniques. Second, we will observe the metallicity gradient of M31 via RSG spectroscopy using MOSFIRE on Keck while collaborators led by Ben Davies will conduct a study of RSGs in the SMC, LMC, and NGC300 using their access to the ESO. With the addition of these two populations our team will have data on sub-solar (SMC and LMC), solar (Per OB-1), and super-solar (M31) metallicity RSG populations. The proposal for two nights on Keck will likely be submitted for the 2012B semester. Finally, with a solid calibration in hand and a proof of concept observation of the metallicity gradient in M31, we will stretch to even greater distances using MOSFIRE on Keck: out to ∼4 Mpc with an observation of M81 or NGC2403. As this work progresses we will assess the applicability of the developed J band methodology to super star clusters (SSCs, or young massive clusters; YMCs) as these objects are dominated in the J band−after 9 Myrs−by RSG members. If successful, additional science will be conducted using SSCs at even greater distances; for example, M51 at 8.4 Mpc contains thousands of such clusters. In the proposal which follows, we describe the technical details regarding our new J band diagnostics (§2), how these diﬀerent stages of calibration and science application will be implemented (§3), and a basic schedule for the completion of this work over the next three years (§4).

5 2. Analysis Techniques and Technical Considerations

2.1. Finding RSGs: Target Selection

The detection of RSGs in galaxies beyond the Local Group is straightforward and efficient. To illustrate this, Fig.6a displays Large Magellanic Cloud color-magnitude and color-color diagrams at different pass-bands demonstrating the clear separation of RSGs from other objects. In the case of the LMC, the contamination with red foreground objects is about 40% at optical wavelengths, but is less towards the IR. When observing galaxies at larger distances the contamination will be strongly reduced because of the smaller angular sizes compared to the LMC and the significantly lower brightness of the target RSGs. Fig.6b shows a corresponding diagram obtained for NGC 300 (1.9 Mpc) using HST ACS (Bresolin et al., 2005). We note that existing HST data, in particular taken by the ANGST survey (Dalcanton et al., 2009) will provide data for many galaxies. Additional HST observations and targeted ground based observations with wide field IR imagers (CFHT, VISTA) plus ongoing surveys such as PS1 and in the future JWST will always be an option.

2.2. Modeling RSGs

Over the course of this thesis a set of algorithms will be developed to apply to J band specta and sets of models. These methods will likely utilize a hybrid principal component analysis and χ2 algorithm using Monte Carlo Markov Chain techniques and nonlinear interpolation between a coarse model grid in order to most effectively derive modeled parameters and assess the significance of those results. As a proof of concept a rudimentary χ2 method has been developed for this proposal, roughly following the techniques of Davies et al. (2010). The method in general successfully recovers existing effective temperature and metallicity ([Z]) values for a number of RSGs from SpeX archival data (Rayner et al. 2009) using the MARCS model grid (Gustafsson et al. 2008). Table 1 and Figure 4 present the results of this initial analysis.

The MARCS grid is computed with assumptions of local theromdynamic equilibrium (LTE) and spherical geometry and has worked well in the analysis of RSG atmospheres Davies et al. (2010). We are collaborating to develop a more extended and detailed grid of MARCS models applicable to the RSGs. Over the course of this work we will explore other solutions to modeling the RSG atmospheres with greater physical accuracy, including non-LTE calculations and considerations for internal mixing and its eﬀects, mass-loss and convection. But the critical point in this regard is that Davies et al. (2010) recover the correct [Z] metallicity values in their study of archival SpeX observations of RSGs using the existing MARCS grid. Our rudimentary techniques are consistent with those results. This oﬀers the “proof of concept” for utilizing low resolution spectroscopy in the J band towards our proposed science goals.

2.3. The Enormous Potential of Quantitative Spectroscopy of RSGs in Other Galaxies

At present, for extragalactic spectroscopic RSG studies the two instruments of choice are the IR multi-object spectrographs KMOS at the ESO VLT and MOSFIRE at Keck, which will become

6 (a) (b)

Fig. 3.— Identifying RSGs in other galaxies. (a) Photometry of LMC stars, with spectral types indicated where known. RSGs congregate in a locus separated from other stars. The separation becomes more pronounced towards IR wavelengths. Areas within the dotted lines contain more than 60% RSGs. At larger distances the discrimination improves (see text). Figure from Davies and Kudritzki, priv. comm. (b) CMD (I vs. V-I) of one out of six HST ACS ﬁelds in NGC 300 (1.9 Mpc). The RSGs are easily identiﬁed as the brightest objects at V-I ' 2. (Data from Bresolin et al. 2005).

7 Mg I Si I 1.2 K I Ti I 1.1 Fe I

Flux 1.0

0.9

0.8 1.15 1.17 1.19 1.21 1.23 Wavelength [microns]

Fig. 4.— J band observation of the galactic RSG HD39801 observed with SpeX on IRTF (Rayner et al. 2009) and ﬁt with the MARCS model grid. Black points represent the observed spectrum, while the red line plots the bet ﬁt (lowest χ2) model. The strongest lines are marked with arrows.

Table 1.

Davies et al. 2009 This Work

NAME SpT T [K] Tfit [K] log[Z] Tfit [K] log[Z]

HD 187238 K3Iab-Ib 4015 3630 ± 180 0.04 ± 0.21 3840 ± 260 0.11 ± 0.32 HD 185622A K4Ib 3928 3590 ± 170 0.12 ± 0.20 3820 ± 250 0.16 ± 0.29 HD 216946 K5Ib 3840 3580 ± 230 -0.15 ± 0.27 3770 ± 240 0.00 ± 0.37 HD 14404 M1-Iab-Ib 3745 3580 ± 170 0.10 ± 0.19 3800 ± 250 0.14 ± 0.30 HD 39801 M1-M2Ia-Ia 3710 3520 ± 160 0.19 ± 0.21 3750 ± 220 0.16 ± 0.28 BD+60 265 M1.5Ib 3710 3570 ± 140 0.16 ± 0.17 3810 ± 240 0.17 ± 0.29 HD 14469 M3-M4Iab 3550 3530 ± 160 0.15 ± 0.22 3740 ± 220 0.08 ± 0.33

Note. — Temperatures are taken from literature spectral type and temperature scale of Levesque et al. (2005). Error bars are derived from scatter in the best models weighted by χ2 goodness of ﬁt. Note the good agreement with Davies et al. (2010) and existing temperature scale. Increased values for log[Z]

and σlog[Z] in this work are due, in part, to a singular value of turbulence (2 km/s) while Davies et al. (2010) utilize a grid with multiple values for turbulence (2-5 km/s).

8 available in 2011. For the J-band, assuming a spectral resolution of R=3000, a signal-to-noise of S/N=50, a seeing of 0.7 arsec and an airmass of 1.1 Davies et al. (2010) have estimated a limiting magnitude of 19 mag for a sequence of exposures totalling two nights. For a galaxy at a distance of 4 Mpc and the range of absolute J-band magnitudes given above we would be able to observe all

RSGs brighter than MJ =-9 mag, which corresponds to objects with an initial mass on the main sequence larger or equal to 14M . For a galaxy such as the Milky Way this would give us over 1000 stars distributed over the disk of the galaxy to choose from. With somewhat longer exposure one could go to further distances or reach lower masses as well. For galaxies at signiﬁcantly larger distances we can also concentrate on the brightest objects with MJ ' -11 mag, for which we still expect more than 100 objects per galaxy.

With the future ELTs much larger distances can be reached. Very recently, Evans, Davies, Kudritzki et al. (2010) using the E-ELT simulator for the multi-object IFU spectrograph EAGLE and including the dramatic eﬀects of adaptive optics found that in a 10 hour exposure we will be able to reach J=24 mag with a S/N suﬃcient to determine chemical abundances with an accuracy of ' 0.1 dex. For RSG absolute magnitudes range between MJ = -8 to -11 mag this will give us limiting distances between 25 to 100 Mpc. This opens up a remarkable volume of the local universe for detailed and precise studies of the formation and chemical evolution of entire galaxy clusters, such as those of Virgo, Fornax, Puppis, Eridanus, Pegasus, Pisces, Cancer, Perseus and even Coma.

An additional very exciting perspective is the the use of super-star clusters (SSCs), also called young massive clusters (YMCs), as cosmological abundance indicators through low resolution J- band spectroscopy. These objects contain many dozens of RSGs, which dominate the integrated J-band ﬂuxes after 9 Myrs (Gazak, Davies, Kudritzki, in prep). Quantitative spectral analysis using cluster population synthesis techniques may, therefore, allow for the determination of chemical abundances at much larger distances than possible with individual RSGs. To develop this technique will also be part of this proposal.

9 3. Research Plan

3.1. “Solar Neighborhood”: Testing J Band Techniques in the Milky Way

While initial tests of chemical enrichment from modest resolution J-band spectroscopy are promising (Davies et al. 2010), rigorous tests in the solar neighborhood will allow for direct comparison to existing high resolution H and K band studies as well as chemical compositions derived from well calibrated techniques applied to optical spectroscopy of blue supergiants (such as Ku- dritzki et al. 2008). Thus the first step in this dissertation work will include observations of Milky Way RSGs for the goal of exposing any necessary calibrations for applying the technique to previously unobserved target populations. Furthermore, we must be certain that any systematic effects of the new RSG analysis methods (or the existing BSG techniques) are located and understood. For this we will collaborate with Miguel Urbaneja who has collected and is analyzing the full population of Perseus OB-1 BSGs using existing and well refined techniques.

This collaboration will assure that metallicities derived from RSGs and BSGs are consistent and accurately represent the target populations−an invaluable scientiﬁc result when the techniques are applied to more distant populations. Calibrations may be required due to the diﬀerence between the full nLTE model codes for BSGs and the less complex LTE grids available for RSGs. As new RSG atmosphere codes become available with more complex features such as nLTE calculations and interior mixing (the driver of convection and mass-loss), this data set can be reused to test and calibrate the new models. For the construction of this set of calibrations we will target a population of RSGs in the Milky Way’s Perseus OB-1 association (Table 2). Collecting high resolution (R∼20,000) J and H band spectra using IRCS on Subaru (1N, 2011B) and low resolution (R∼ 2,000) 0.8-2.5 µm (covering J, H, and K band) spectroscopy using SpEX on IRTF (1N, 2011B) will allow for a direct comparison between existing high resolution and newly proposed low resolution techniques. The target list covers spectral types K5 to M5, and the Perseus OB1 population metallicity is roughly solar. Total exposure times required (including overheads) will be 10-20 minutes per object.

In addition, by observing the full population of RSGs in Per OB-1, we will be able to test the feasibility of population synthesis star cluster work (see §3.3.2), as we know the mass, age, and distance of Per OB-1 (Currie et al. 2010). With spectra of all RSGs in Per OB-1, an integrated light J-band spectrum can be constructed and used as a test of techniques developed to recover metallicity and age information from unresolved populations.

Our collaborators in Europe will, in the meantime, conduct a similar pilot study of RSGs in the Large and Small Magellanic Clouds (LMC, SMC). This will provide the ﬁrst science results on an extragalactic and signiﬁcantly sub-solar metallicity population. The data collected for this portion will be strictly low resolution, and will require any calibrations developed by our Perseus OB-1 work.

10 Table 2. Perseus OB-1 Red Supergiants

Identiﬁer mV mJ mH SpT

BD+59 372 9.30 5.33 4.20 K5Ia BD+56 595 8.18 4.13 3.22 M0Iab BD+57 530A 9.19 4.57 3.52 M0Iab HD 14580 8.40 4.48 3.46 M0Iabvar HD 14330 7.90 3.80 2.91 M1Iab HD 14404a 7.84 3.56 2.68 M1Iab HD 13658 8.95 4.72 3.66 M1Iab HD 236947 8.72 4.02 3.13 M2Ia HD 13136 7.75 3.00 2.14 M2Iab HD 14826 8.24 3.47 2.47 M2Iab HD 236915 8.29 4.03 2.97 M2Iab HD 14242 8.36 3.95 3.04 M2Iab BD+58 501 9.37 3.69 2.44 M2Iab HD 236979 8.10 3.26 2.30 M2Iab BD+57 647 9.38 3.83 2.71 M2Iab HD 14270 7.80 3.38 2.48 M2.5Iab BD+56 724 8.70 3.10 2.00 M3Iab HD 14469a 7.55 2.82 1.93 M3.5Iab BD+56 512 9.20 3.68 2.68 M4Ib HD 14528 9.17 2.95 1.85 M4Iaevar HD 14488a 8.50 3.05 2.11 M4.5Iab

aExisting SpeX Obs (Rayner et al. 2009)

Note. — Target list for calibration of low resolution J band RSG metallicity extraction. Target selection and V mag from Garmany & Stencel 1992. J and H magnitudes from 2MASS (Skrut- skie et al. 2006).

11 3.2. “Galactic Neighborhood”: the Local Group Galaxies M31 and M33

A well calibrated and thoroughly tested set of elemental composition analysis tools for RSGs will be immediately applicable to M31 and M33, two large spiral galaxies in the local group. While collaborators with access to southern telescopes secure observations of LMC and SMC, we will propose for two nights on Keck using the new Mid IR spectroscope MOSFIRE to observe a selection of ∼80 RSGs across the radial gradient of M31. Radial abundance gradients in large spirals remain a key target for understanding galaxy formation and evolution and provide important input to modeling the evolution of these galaxies. To date, very little spectroscopic information exists over the radial gradient of M31, a nearby Milky Way analog. Those measurements are derived from H ii regions whose analysis is plagued by signiﬁcant calibration issues. Observations of individual stars are critical to provide better spatial resolution and introduce observational constraints to allow for better use of data on H ii regions, as well as providing a rich additional source of information on α/Fe element ratios.

M33, the Sc galaxy companion of M31, is a target which will benefit significantly from observations of RSGs. This galaxy has been studied by many teams (Searle 1971; Vilchez et al. 1988; Garnett et al. 1997; Willner & Nelson-Patel 2002; Magrini et al. 2007; Rosolowsky & Simon 2008; U et al. 2009). These observations provide the input required to make modeling of the chemical evolution of M33 second in detail only to work with the Milky Way (Magrini et al. 2009). However, despite this progress, there exists no consensus on the slope of the radial O/H abundance gradient. Different published results range from consistent with no gradient (Rosolowsky & Simon 2008) to a steep gradient of -0.11 ± 0.02 dex kpc−1 (Vilchez et al. 1988; Garnett et al. 1997)–and everywhere in between (Willner & Nelson-Patel 2002; Magrini et al. 2007). Recent work by U et al. (2009) has measured a modest gradient of -0.07 ± 0.01 dex/kpc using accurate BSG methods. Additional observations of BSGs are currently being analyzed to clarify this gradient. The observation of an independent set of stars will aide greatly the scientific discussion of M33. As a nearby spiral, it is important to understand whether M33 is a unique case which cannot be explained by current theoretical work on galaxy evolution or if previous work is flawed or biased by calibration techniques. By utilizing the brigtest RSGs we can cover the radial extent of M33 in one night using MOSFIRE on Keck.

3.3. “Out of Town”: Beyond the Local Group

With plans to construct the next generation of 30m-class telescopes the applicability of extragalactic stellar methods in the IR becomes increasingly attractive, as entire clusters of galaxies will be available for study through their individual stellar components. However, even with existing technology on 8m class telescopes we can begin to reach to indivual stars in galaxies beyond the local group and even further by utilizing simple stellar population synthesis to study super star clusters (SSCs) at distances from 25 to 100 Mpc.

12 3.3.1. Abundance Gradients of M81 and NGC 2403

The first sensible galaxy to target using this technique is M81, a massive Sb spiral galaxy at 4 Mpc. Observations of H ii regions using the strong line methods indicate super-solar metallicity and a very shallow abundance gradient (Garnett & Shields 1987). These characteristics−usually attributed to the considerable mass of M81−require confirmation by work with M81’s supergiant population. This makes M81 a salient target as the results will allow for critical constraints on the modeling of galaxy evolution as a function of total mass. In particular, past work with H ii region hints that the abundance gradients in spiral galaxies are independent of mass when considered on a dimensionless length scale in units of disk scale length (Garnett et al. 1997; Skillman 1998; Garnett 2004). If confirmed by more accurate supergiant methods this would represent a fundamental constraint on galaxy evolution (Prantzos & Boissier 2000). Our team has already collected and are analyzing optical spectra of ∼40 BSGs in M81.

A second important target is the Scd spiral NGC 2403 in the M81 group. A study of 12 H ii regions suggests a radial abundance gradient and nearly solar central metallicity (Garnett et al. 1997). This represents a factor of three lower metallicity than M81, and an important target for testing the previously mentioned constraints on galaxy abundance gradients as a function of mass. Three nights with LRIS on Keck have been secured in early 2011 to collect optical spectra of a target population of BSGs in NGC 2403.

Within the structure and timeline of a thesis project it makes sense to select one of these galaxies for observation. While both (and many more) will be observed as the techniques continue to evolve, the selection of a target for this thesis will be made at a later date based on the observing semester and work to date. It is important to note that each target has previous H ii region observations while results from BSGs will soon be available. Thus for each target the observations of RSGs will represent an important step for testing the previous studies with an independent technique applied to an independent target population. MOSFIRE will again represent the choice instrument for this work.

3.3.2. Super Star Clusters

Super star clusters (SSCs) or young massive clusters (YMCs) are dense aggregates of young stars found in star forming galaxies. First studied in detail by Arp & Sandage (1985) and later conﬁrmed as clusters of stars with HST by O’Connell et al. (1994), these objects have since been studied and a wealth of literature has been developed based on broad band photometry and spectroscopy (see Portegies Zwart et al. (2010) for a review). In general, the spectroscopic results focus on high resolution H band observations and thus suﬀer from the same drawbacks as observations of RSGs.

5 We have simulated the coeval evolution of a 10 M SSC using Geneva isochrones (Meynet & Maeder 2005) and found that the J band spectrum of a SSC is dominated by the presence of roughly ﬁfty RSGs as soon as the clusters are older than 9 Myrs (Fig 5). Due to this we predict that the J band spectrum can be considered a synthesis of the RSG population with a continuum

13 adjustment to account for the small percentage of “contaminative” ﬂux. Thus observable distances are extended by a factor of 10, providing immediate access to study of RSG-dominated SSCs out to 100 Mpc and well beyond with an ELT (Gazak, Davies & Kudritzki 2010, in prep). The work has shown excellent agreement to existing photometric data on SSCs in M51 (see Fig. 5, Bastian et al. 2005).

We plan to observe a few test SSCs using time remaining on our initial work with Perseus OB-1 RSGs. With Subaru we will collect high resolution spectra in J and H band to resemble previous studies as well as at lower resolution with remaining time on IRTF. At mJ ∼ 13 exposure times of 1h total are required, meaning that we will be able to observe one or two SSCs in NCG 1569 using available time. These spectra (along with our complete population observations of Per. OB-1 RSGs) will allow us to test our simulations and hopefully model and extract information on the metallicity and chemical abundances of the SSC environment. If successful–that is that the systems can be modeled as a coeval population and that J band results at low resolution match higher resolution H and K band observations–we will plan to use IRTF with SpeX to continue to collect spectra on SSCs of varying ages and metallicities in NGC 1569 and NGC 6946. The required exposure times with overhead should require 3 nights of SpeX time for the ∼15 SSCs with mJ ≤ 14 in NGC 1569. The next logical step in this work would be to reach even further to observe SSCs in M51 at 8.4 Mpc. From unpublished HST photometry provided by collaborator Nate Bastian we have identiﬁed roughly 20 clusters with 14 ≤ mJ ≤ 19 mag, for which MOSFIRE would be required. Observing these clusters could provide signiﬁcant information regarding chemical evolution of M51 but will be pursued within the context of this thesis only if time allows.

5 Fig. 5.— Monte-carlo population synthesis simulation of a 10 M super-star cluster using Geneva-group evolutionary tracks at solar metallicity, which include the effects of stellar rotation (error bars give the statistical uncertainties of the simulation). Left: J-band flux contribution of RSGs in percent of the total J-band flux as a function of cluster age. Center: Number of RSGs as a function of cluster age. From Gazak, Davies & Kudritzki, 2010, in prep. Right: V-J SSC color as a function of cluster age. Filled red squares represent unpublished data from collaborator Nate Bastian (similar to Bastian et al. (2005)) of SSCs in M51. 5 Black circles with error bars show the theoretical V-J age for the 10 M cluster which we simulate. The agreement is remarkable considering that the Bastian data cover many independent SSCs of varying mass, and hints strongly that this type of population synthesis modeling of SSCs can be very successful.

14 4. Technical and Scheduling Details

The completion of the full proposed thesis will likely require 5 nights on Keck I with MOSFIRE, 1-2 nights on Subaru with IRCS, and roughly 10 nights on IRTF with SpeX. This represents a significant investment of telescope time on some of the finest instruments in the world, and an ambitious amount of work for a dissertation project. We are aware of−and not concerned by−these points. This work will produce results applicable to the base assumptions of a wide range of disciplines, from the study of the atmospheres of RSGs to galaxy formation and evolution to the chemical enrichment history of the universe. The techniques developed are the basis of an entirely new field of extragalactic work, complementary to recent BSG methods and more applicable to future instruments with massive collecting areas and adaptive optics optimized for the infrared wavelengths. We will work as part of a healthy and active global team and benefit from open collaboration as these members work to acquire similar results from targets in the southern hemisphere. Finally, the work for this thesis is cohesive but well segmented such that failures due to instrument and weather issues will not represent a failure or require significantly extending the timeframe. While each section represents an important step, this is work which will continue beyond the PhD and need not necessarily fold into that document. Based on successful proposed observations we expect a minimum of 5 publications over the next three years (the J band potential of SSCs, calibrating the RSG method, M31, M81 or NCG2403, and J band observations of SSCs).

Because MOSFIRE has not yet been released for science, we note that there are potential backup instruments available, including FMOS on Subaru and NIRSPEC on Keck. In the worst case scenario, where MOSFIRE does not become available during the period of this thesis, we will propose for M31 and M33 using NIRSPEC and/or FMOS and drop a galaxy beyond the local group. Limitations for these instruments are as follows: FMOS has a slightly inferior resoluton of R ∼ 2200, and NIRSPEC is a single object instrument.

15 Table 3. Thesis Timeline

Semester Proposals

2010B-2011A Propose thesis, TAC pre-review 2011B Perseus OB1 and SSC test (1n Subaru, 1n IRTF) M31 (MOSFIRE dependent) 2012A SSC observations (0-3n IRTF or 2n IRCS Subaru) 2012B M31 and/or M33 (2-3n Keck I MOSFIRE or Keck II NIRSPEC) 2013A M81 or NGC 2403 (3n Keck I) 2013B SSC observations (0-3n IRTF or 2n IRCS Subaru) 2014A Defend thesis

Totals: 5n Keck, 1n Subaru, 1-7n IRTF

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This preprint was prepared with the AAS LATEX macros v5.2.