The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters

By Scott C. Trager, S. Faber, G. Worthey, J. J. G. Ociw, U. Observatory, St. Ambrose, IA-UNAM. 2000

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This paper commences a series of investigations into the stellar populations of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evolution histories of present-day elliptical galaxies. The primary galaxy sample analyzed is that of González, which consists of 39 elliptical galaxies drawn primarily from the local field and nearby groups, plus the bulge of Messier 31. Single-burst stellar population (SSP)–equivalent ages, metallicities, and abundance ratios are derived from Hβ, Mg b, and ⟨Fe⟩ line strengths using an extension of the Worthey models that incorporates nonsolar line-strength "response functions" by Tripicco & Bell. These functions account for changes in the Lick/IDS indices caused by nonsolar abundance ratios, allowing us to correct the Worthey models for the enhancements of Mg and other α-like elements relative to the Fe-peak elements. SSP-equivalent ages of the González elliptical galaxies are found to vary widely, 1.5 Gyr ≲ t ≲ 18 Gyr, while metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked around ⟨[Z/H]⟩ = +0.26 and ⟨[E/Fe]⟩ = +0.20 (in an aperture of radius re/8). The enhancement ratios [E/Fe] are milder than previous estimates because of the application of nonsolar abundance corrections to both Mg b and ⟨Fe⟩ for the first time. While [E/Fe] is usually greater than zero, it is not the "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken ⟨Fe⟩ but also to strengthen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al.), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the E group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age increasing by 25%, metallicity decreasing by ⟨[Z/H]⟩ = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius re/2 for nearly all systems. Our ages have an overall zero-point uncertainty of at least ∼25% because of uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] ≠ 0 on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hβ by blue stragglers or hot horizontal-branch stars, or by fill-in of Hβ by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

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Citations in the corpus

A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies. II. Formation of Red Ellipticals (2007, methodology): “Direct calculation of the inferred stellar population ages from line index and SED fitting (following Trager et al. 2000) for realistic star formation histories from the semianalytic models of Somerville et al. (2001) and the hydrodynamical merger simulations of Robertson et al. (2006b) suggests…”
Evolutionary stellar population synthesis with MILES – I. The base models and a new line index system (2010, methodology): “By far the most popular method is to build key diagnostic model grids by plotting an age-sensitive indicator (e.g. Hβ) versus a metallicitysensitive indicator (e.g., Mgb, 〈Fe〉), to estimate the age and metallicity (e.g. Trager et al. 2000; Kuntschner et al. 2006).”
The ATLAS3D project - XIII. Mass and morphology of H I in early-type galaxies as a function of environment (2011, background): “…early insights by Gunn, Stryker & Tinsley (1981) and Rabin (1982), it is now established that a large fraction of ETGs are forming small amounts of stars or have done so in their recent past (e.g. González 1993; Trager et al. 2000; Yi et al. 2005; Kaviraj et al. 2007; Thomas et al. 2010).”
Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models (2015, methodology): “…indicator (e.g. Mgb, 〈Fe〉), to estimate these parameters (e.g. Rose 1994; Worthey 1994; Vazdekis et al. 1996; Kuntschner et al. 2006; Schiavon 2007), including estimations of the abundance ratios of individual elements (e.g. Trager et al. 2000; Thomas, Maraston & Bender 2003a; Schiavon 2007).”
Galaxy Luminosity Functions to z~1 from DEEP2 and COMBO-17: Implications for Red Galaxy Formation (2005, background): “Third, the mean light-weighted stellar-population ages of local Es scatter widely, from over 10 Gyr down to just a few Gyr (e.g., Gonzalez 1993, Trager et al. 2000a, Jørgensen 1999, Terlevich & Forbes 2002), with most be- 23 Our use of the word age here is not meant to obscure the fact that mass…”
A comparison of stellar populations in galaxy spheroids across a wide range of Hubble types (2002, methodology): “Correlations between indices sensitive to α-elements (e.g. Mg2 and Mgb) and velocity dispersion have been widely observed in early-type galaxies (Bender et al. 1993; González 1993; Jørgensen 1997; Bernardi et al. 1998; Concannon et al. 2000; Kuntschner 2000; Trager et al. 2000b).”
Stellar populations of early-type galaxies in different environments - II. Ages and metallicities (2006, methodology): “However, some authors, (e.g. Trager et al. 1998, 2000b; Ferreras et al. 1999; Jørgensen 1999) have studied the scalere- P. Sánchez–Blázquez et al.: Stellar populations of early-type galaxies in different environments II 9 Table 1.”
Spatially resolved spectroscopy of Coma cluster early-type galaxies III. The stellar population gradients ? (2003, methodology): “The impact from element ratio changes is computed with the help of the Tripicco & Bell (1995) response functions, using an extension of the method introduced by Trager et al. (2000a).”
The ATLAS3D Project – XXX. Star formation histories and stellar population scaling relations of early-type galaxies (2015, methodology): “The smallest of these is comparable to aperture sizes commonly used in conventional long-slit investigations (e.g. Trager et al. 2000; Thomas et al. 2005).”
UV-Optical Colors as Probes of Early-Type Galaxy Evolution (2006, methodology): “We find, in this case, that galaxies are generally more metal-poor than the mean relations derived by Trager et al. (2000a) - increasing B simply exacerbates this situation.”
Hot Gas In and Around Elliptical Galaxies (2003, background): “For the much smaller cooling Ṁ in E galaxies and their groups the evidence for star formation is less obvious such as the features in optical spectra from a “frosting” of young stars (Mathews & Brighenti 1999a; Trager et al. 2000).”
NOAO Fundamental Plane Survey. II. Age and Metallicity along the Red Sequence from Line-Strength Data (2005, methodology): “Although Trager et al. (2000b) do not claim a trend of age with σ, a simple fit to their data yields an age-σ trend with an exponent of 0.6± 0.2, consistent with our results.”
The Recent and Continuing Assembly of Field Elliptical Galaxies by Red Mergers (2005, background): “…is general agreement that themean age is high, there are strong indications that not all stars in all elliptical galaxies formed at high redshift; Trager et al. (2000) and others (e.g., Yi et al. 2005) find evidence for a ‘‘sprinkling’’ of recent star formation in otherwise old ellipticals, and…”
The SAURON project – VIII. OASIS/CFHT integral-field spectroscopy of elliptical and lenticular galaxy centres (2006, background): “We perform a robust straight-line fit to the data, the parameters ofwhich are: log t = −3.95 + 2.04 log σ, (3) [Z/H ] = 0.02 + 0.14 log σ, (4) [α/Fe] = −0.45 + 0.26 log σ. (5) As found by many authors (e.g. Carollo, Danziger, & Buson 1993; Trager et al. 2000; Kuntschner et al. 2001; Proctor & Sansom”
Near-infrared line-strengths in elliptical galaxies: evidence for initial mass function variations? (2002, background): “…the existence of an apparent spread of mean ages (González 1993; Faber et al. 1995; Jørgensen 1999) and element abundances ratios (Worthey 1998; Trager et al. 2000a), suggesting a variety of interpretations of scaling relations like the colour– magnitude or Mg2–σ relations (Bender, Burstein…”
The Cosmological Constant and Dark Energy (2002, background): “…populations, and some elliptical galaxies have grown by recent mergers, as predicted in the CDM model.66 But prominent merger events are rare, and the young stars seen in ellipticals generally seem to be a “frosting” (Trager et al., 2000) of recent star formation on a dominant old star population.”
DISSIPATION AND EXTRA LIGHT IN GALACTIC NUCLEI. IV. EVOLUTION IN THE SCALING RELATIONS OF SPHEROIDS (2008, background): “…bands: if, say, a low-mass system atz = 0 has a stellar population age of∼ 7Gyr whereas a high-mass system has an age of∼ 12Gyr, as observed (see e.g. Trager et al. 2000; Nelan et al. 2005; Thomas et al. 2005; Gallazzi et al. 2006), then inverting passive evolution toz = 0.7, the lowmass (younger)…”