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The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies

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The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies Accepted to the Astrophysical Journal on Tuesday, October 1, 2013 Preprint typeset using LATEX style emulateapj v. 5/2/11 THE UNIVERSAL STELLAR MASS–STELLAR METALLICITY RELATION FOR DWARF GALAXIES Evan N. Kirby1,2, Judith G. Cohen3, Puragra Guhathakurta4, Lucy Cheng5,6, James S. Bullock1, Anna Gallazzi7,8 Accepted to the Astrophysical Journal on Tuesday, October 1, 2013 ABSTRACT We present spectroscopic metallicities of individual stars in seven gas-rich dwarf irregular galaxies (dIrrs), and we show that dIrrs obey the same mass–metallicity relation as the dwarf spheroidal 0.30±0.02 (dSph) satellites of both the Milky Way and M31: Z∗ ∝ M∗ . The uniformity of the relation is in contradiction to previous estimates of metallicity based on photometry. This relationship is roughly continuous with the stellar mass–stellar metallicity relation for galaxies as massive as M∗ = 12 10 M⊙. Although the average metallicities of dwarf galaxies depend only on stellar mass, the shapes of their metallicity distributions depend on galaxy type. The metallicity distributions of dIrrs resemble simple, leaky box chemical evolution models, whereas dSphs require an additional parameter, such as gas accretion, to explain the shapes of their metallicity distributions. Furthermore, the metallicity distributions of the more luminous dSphs have sharp, metal-rich cut-offs that are consistent with the sudden truncation of star formation due to ram pressure stripping. Subject headings: galaxies: abundances — galaxies: fundamental parameters — galaxies: dwarf — galaxies: irregular — Local Group 1. INTRODUCTION which dilute the metallicity of both the gas and the stars The average metal content of a galaxy correlates with that form from the gas. Yet another explanation for the its mass. More massive galaxies are more metal-rich than mass–metallicity relation is a stellar initial mass func- less massive galaxies. The relation can be explained by tion (IMF) that changes with the rate of star forma- the retention of metals in the galaxies’ gravitational po- tion (K¨oppen et al. 2007). Because that rate depends on tential wells (e.g., Dekel & Silk 1986). High-mass galax- galaxy mass and because the metal yield depends on the ies have deep potential wells that can resist some of the masses of stars, a galaxy’s metallicity then depends on expulsion of gas and metals by supernova winds, stel- its stellar mass. lar winds, and galaxy-scale feedback. Low-mass galaxies Galactic metallicity is typically measured in the lack the gravity to resist these feedback mechanisms. The gas phase. Emission line diagnostics of metallic- ity (e.g., Kewley & Dopita 2002) sample the metallic- correlation between metallicity and mass can also be ex- II plained by a correlation between star formation efficiency ity of H regions. Hence, these measures probe and stellar mass (e.g., Matteucci 1994; Calura et al. presently star-forming gas. Using strong line diagnostics, 2009; Magrini et al. 2012; Pipino et al. 2013). If mas- McClure & van den Bergh (1968) and Lequeux et al. sive galaxies evolve quickly, then they can achieve high (1979) established the first luminosity–metallicity rela- stellar masses and low gas mass fractions. Consequently, tions (LZRs) for star-forming galaxies. Lequeux et al. their metallicities will be high. On the other hand, slowly found that the effective metal yields of all of the galax- evolving, low-mass galaxies can have high gas fractions, ies they considered were below the true yield expected from a “closed box” or “simple” model of galactic chem- * The data presented herein were obtained at the W. M. Keck ical evolution (Schmidt 1963; Talbot & Arnett 1971; Observatory, which is operated as a scientific partnership among Searle & Sargent 1972). More recently, Tremonti et al. the California Institute of Technology, the University of Cali- (2004) showed that the average metallicities of star- arXiv:1310.0814v2 [astro-ph.GA] 16 Oct 2013 fornia and the National Aeronautics and Space Administration. forming galaxies in the Sloan Digital Sky Survey (SDSS, The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Abazajian et al. 2004) correlate strongly with their stel- 1 University of California, Department of Physics and As- lar masses or rotation speeds. The correlation with tronomy, 4129 Frederick Reines Hall, Irvine, CA 92697, USA, stellar mass has an intrinsic scatter of only 0.1 dex in [email protected] 2 Center for Galaxy Evolution Fellow. log(O/H). As with previous studies, Tremonti et al. in- 3 California Institute of Technology, Department of Astron- terpreted the relation as a progression of a larger effective omy & Astrophysics, 1200 E. California Blvd., MC 249-17, yield for more massive galaxies. Expressed another way, Pasadena, CA 91125, USA more massive galaxies lose a smaller fraction of the met- 4 UCO/Lick Observatory and Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, als that their stars produce. USA The mass–metallicity relation (MZR)—where metallic- 5 The Harker School, 500 Saratoga Ave., San Jose, CA 95117, ity was measured in the gas phase—extends down to the USA mass range of dwarf galaxies as small as a few million 6 Harvard University, Department of Astronomy, 60 Garden St., Cambridge, MA 02138, USA solar masses. Mould et al. (1983) and Skillman et al. 7 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi (1989) showed that dwarf elliptical (dE) and dwarf ir- 5, I-50125 Firenze, Italy 8 regular galaxies (dIrrs) in and around the Local Group Dark Cosmology Centre, Niels Bohr Institute, University of (LG) obey a LZR. Garnett (2002) extended the rela- Copenhagen, Juliane Mariesvej 30, 2100 Copenhagen, Denmark 2 Kirby et al. tion to more distant spiral galaxies. Irregular and spiral of gas metallicity with time, the gas-phase metallicity galaxies obey the same, unbroken relation over 4.5 orders is greater than the average stellar metallicity. Further- of magnitude in luminosity. more, the gas-phase metallicity in principle fluctuates One of the sources of error in determining the MZR is more than the stellar metallicity. The gas metallicity the stellar mass-to-light ratio (M∗/L). This ratio is re- changes as rapidly as gas flows into and out of the galaxy, quired to convert the LZR into the MZR. It depends on a whereas the stellar metallicity responds to gas flows on a galaxy’s star formation history (SFH) with potential con- longer timescale, which depends on the SFR. On the tributions from the stellar IMF. Lee et al. (2006a) mea- other hand, Berg et al. (2011, 2012) showed that the sured stellar masses for dwarf galaxies based on 4.5 µm dispersion in the MZR is just 0.15 dex when the “di- luminosity, which is less sensitive to age, SFH, and dust rect” method is used to measure gas-phase abundances than visible luminosity. While Skillman et al. (1989) al- from auroral lines. Furthermore, rare outliers from the ready established that the LZR applies to dwarf galaxies, MZR when using the strong nebular lines are not out- Lee et al. (2006a) showed that the MZR from more mas- liers when using the more trusted, direct method. The sive spiral galaxies (Tremonti et al. 2004) also applies to low dispersion leaves little room for large fluctuations in dIrrs. the gas-phase metallicity. However, the direct method is The MZR also exists at high redshift. Erb et al. (2006) only practical for a limited range of galaxies because the found that the relation persists, but evolves in the sense [O III] λ4363 emission line is intrinsically faint, especially that high-redshift galaxies are more metal-poor at a at high metallicities (12 + log(O/H) & 8.5). given stellar mass than galaxies in the local universe. Quiescent and gas-free galaxies have no emission lines. Zahid et al. (2013) and Henry et al. (2013) showed that The only light from these galaxies comes from stars. the MZR evolves smoothly from z = 2.2 to the present. Hence, spectroscopic metallicities must be measured However, Mannucci et al. (2010), Hunt et al. (2012), and from absorption lines rather than emission lines. In prac- Lara-L´opez et al. (2013) found that the independent tice, models of the integrated light spectrum from an en- variable controlling the offset of the MZR was star for- tire stellar population (Tinsley & Gunn 1976) are com- mation rate (SFR), not redshift. Because SFR increases pared to observed galaxy spectra. One implementation with redshift (Madau et al. 1996), it appeared as though of this technique is to measure spectrophotometric in- the MZR was evolving, but it is in fact constant after dices, such as Lick indices (Faber 1973; Worthey 1994). the correction for SFR. Correcting for SFR leads to the Bruzual & Charlot (2003) updated the use of line indices unevolving “fundamental metallicity relation.” Star for- to rely on spectral models rather than fixed-resolution mation depresses the gas-phase metallicity of a galaxy templates. This method can even be used with ultra- because star formation requires hydrogen gas. Metallic- violet line indices for high-redshift galaxies (Rix et al. ity is expressed as the ratio of metals to hydrogen. There- 2004; Halliday et al. 2008; Sommariva et al. 2012). Spec- fore, vigorously star forming galaxies contain a lot of hy- tral models can even be compared directly to observed drogen, which dilutes the metals. In fact, Bothwell et al. spectra without compressing the abundance informa- (2013) established that the offset in the MZR correlates tion into a few line indices (e.g., Conroy et al. 2009). better with H I gas mass than SFR. Gallazzi et al. (2005) applied the Bruzual & Charlot The majority of stellar mass in the local universe models to SDSS spectra of over 40,000 galaxies.
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