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

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