The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations

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

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This paper analyzes single stellar population (SSP)–equivalent parameters for 50 local elliptical galaxies as a function of their structural parameters. The galaxy sample is drawn from the high-quality spectroscopic surveys of González (1993) and Kuntschner (1998). The basic data are central values of SSP-equivalent ages, t, metallicities, [Z/H], and "enhancement" ratios, [E/Fe], derived in Paper I, together with global structural parameters including velocity dispersions, radii, surface brightnesses, masses, and luminosities. The galaxies fill a two-dimensional plane in the four-dimensional space of [Z/H], log t, log σ, and [E/Fe]. SSP age, t, and velocity dispersion, σ, can be taken as the two independent parameters that specify a galaxy's location in this "hyperplane." The hyperplane can be decomposed into two subrelations: (1) a "Z-plane," in which [Z/H] is a linear function of log σ and log t and (2) a relation between [E/Fe] and σ in which [E/Fe] is larger in high-σ galaxies. Velocity dispersion is the only structural parameter that is found to modulate the stellar populations; adding other structural variables such as Ie or re does not predict [Z/H] or [E/Fe] more accurately. Cluster and field ellipticals follow the same hyperplane, but their (σ,t) distributions within it differ. Most Fornax and Virgo cluster galaxies are old, with a only a small sprinkling of galaxies to younger ages. The field ellipticals span a larger range in SSP age, with a tendency for lower σ galaxies to be younger. The present sample thus suggests that the distribution of local ellipticals in the (σ,t) plane may depend on environment. Since the (σ,t) distribution affects all two-dimensional projections involving SSP parameters, many of the familiar scaling laws attributed to ellipticals may also depend on environment. Some evidence for this is seen in the current sample. For example, only Fornax ellipticals show the classic mass-metallicity relation, whereas other subsamples do not. The tight Mg-σ relations of these ellipticals can be understood as two-dimensional projections of the metallicity hyperplane showing it edge-on. At fixed σ, young age tends to be offset by high [Z/H], preserving Mg nearly constant. The tightness of the Mg-σ relations does not necessarily imply a narrow range of ages at fixed σ. Although SSP parameters are heavily weighted by young stars, modeling them still places tight constraints on the total star formation history of elliptical galaxies. The relation between [E/Fe] and σ is consistent with a higher effective yield of Type II SNe elements at higher σ. This might occur if the IMF is enhanced in massive stars at high σ, or if more SNe II–enriched gas is retained by deeper galactic potential wells. Either way, modulating Type II yields versus σ seems to fit the data better than modulating Type Ia yields. The Z-plane is harder to explain and may be a powerful clue to star formation in elliptical galaxies if it proves to be general. Present data favor a "frosting" model in which low apparent SSP ages are produced by adding a small frosting of younger stars to an older "base" population (assuming no change in σ). If the frosting abundances are close to or slightly greater than the base population, simple two-component models run along lines of constant σ in the Z-plane, as required. This favors star formation from well-mixed pre-enriched gas rather than unmixed low-metallicity gas from an accreted object.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

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Citations in the corpus

VLT spectroscopy of globular cluster systems - II. Spectroscopic ages, metallicities and [α/Fe] ratios of globular clusters in early-type galaxies (2005, methodology): “The relative age scale of SSP models appears to be quite stable against the choice of different stellar evolutionary tracks for ages > 1 Gyr (Charlot et al. 1996; Trager et al. 2000a; Maraston 2003).”
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 Effect of Environment on the Ultraviolet Color-Magnitude Relation of Early-Type Galaxies (2006, background): “…to this, the observed levels of -enhancement (Worthey et al. 1992; Carollo et al. 1993; Carollo & Danziger 1994a, 1994b; Kodama &Arimoto 1997; Trager et al. 2000) in many giant ellipticals imply that the initial formation starburst was of a very short duration, less than 1 Gyr (Bruzual &…”
Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios (2010, methodology): “As index definitions have not been modified in the present work, we refer the reader to the literature for a comprehensive discussion of Lick index behaviour with element abundance ratio (e.g. Tripicco & Bell 1995; Trager et al. 2000b; Thomas et al. 2003, 2004; Korn et al. 2005).”
Extragalactic Globular Clusters and Galaxy Formation (2006, methodology): “This is generally the case in earlytype galaxies (e.g., Worthey, Faber, & Gonzalez 1992; Matteucci 1994; Trager et al. 2000; Thomas et al. 2005).”
The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the Fundamental Plane of elliptical and lenticular galaxies (2005, background): “The index[MgFe50] = (0.45 × Mgb + Fe5015)/2 is defined such that it is largely insensitive to abundance ratio variations, while Hβ is only weakly dependent on it (Trager et al. 2000; Thomas, Maraston, & Korn 2004).”
Environment and self-regulation in galaxy formation (2009, methodology): “…galaxies were explicitly derived, no evidence for a correlation with mass, luminosity or velocity dispersion was found (Jørgensen 1999; Kuntschner 2000; Trager et al. 2000a; Kuntschner et al. 2001; Poggianti et al. 2001a,b; Vazdekis et al. 2001; Terlevich & Forbes 2002; Mehlert et al. 2003).”
Ultraluminous Infrared Mergers: Elliptical Galaxies in Formation? (2001, background): “Broadband colors and indices are degenerate in metallicity and age, and it isMg2possible that at Ðxed p young ages are o†set by high metallicities, thus exactly preserving the relationshipMg2-p(Trager et al. 2000a, 2000b).”
Photochemical evolution of elliptical galaxies – I. The high-redshift formation scenario (2003, background): “…of view, we infer the presence ofthe so-called m ass-m etallicity relation from the increasing strength ofthe m etallic absorption lines with increasing velocity dispersion of the galaxy (e.g. Faber et al., 1977; Carollo et al., 1993; G onzalez, 1993; D avies et al., 1993; Trager et al.1998, 2000).”
The role of minor mergers in the recent star formation history of early-type galaxies (2007, methodology): “Note that similar techniques (i.e. using an old SSP to represent the underlying stellar population of ellipticals) have been frequently used by previous studies. e.g. Trager et al. (2000) and Ferreras & Silk (2000).”
Spatially resolved spectroscopy of early-type galaxies over a range in mass (2007, background): “Despite this process, this galaxy follow the same Mg-σ relation that the rest of elliptical galaxies (e.g. Trager et al. 2000a; Sánchez-Blázquez et al. 2006a).”
Higher‐order Balmer line indices in α/Fe‐enhanced stellar population models (2004, methodology): “In the TMB03 models, theα/Fe-enhanced chemical mix at fixed total metallicity is obtained by balancing the increased abundance of the enhanced group with a decrease of the abundances of theelements Fe and Cr (see also Trager et al. 2000a).”
Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age Indicators (2003, methodology): “These corrections are entirely based on the extensive work carried out by Trager et al. (2000a) and byKuntschner et al. (2001).”
Colors, Magnitudes, and Velocity Dispersions in Early-Type Galaxies: Implications for Galaxy Ages and Metallicities (2004, background): “But we are hesitant to take this path because the models do not include an important correlation between velocity dispersion and α-element abundance ratios (Trager et al. 2000a; Thomas, Maraston & Bender 2003; Proctor et al. 2004).”
Mass downsizing and ``top-down'' assembly of early-type galaxies (2006, background): “Indeed, even a low level of star formation (e.g. the “frosting” scenario of Trager et al. 2000) is sufficient to make the color of galaxies too blue for qualifying them as colorselected ETGs, whereas quenching such residual SF should ensure that the galaxies will quickly join the red sequence.”
COUNTING LOW-MASS STARS IN INTEGRATED LIGHT (2011, methodology): “…be contrasted with models such as Bruzual & Charlot (2003), which model spectral energy distributions at solar abundance patterns, and models that consider the effect of age, metallicity, and abundance patterns on the Lick index system (e.g., Trager et al. 2000; Thomas et al. 2003b; Schiavon 2007).”
Empirical calibration of the near-infrared Ca ii triplet — I. The stellar library and index definition (2001, background): “…(like the colour–magnitude or Mg2–σ relations; Bower, Lucey & Ellis 1992; Bender, Burstein & Faber 1993; Pedraz et al. 1999; Terlevich et al. 1999; Kuntschner 2000; Trager et al. 2000b), are some of the main issues in the present debate about the evolutionary status of early-type galaxies.”
What Do We Learn from IRAC Observations of Galaxies at 2 < z < 3.5? (2006, background): “There is ample evidence from local fossil records (e.g. Trager et al. 2000; Lancon et al. 2001; Freeman& Bland-Hawthorn 2002; Förster Schreiber et al. 2003; Angeretti et al. 2005) and high redshift studies (e.g. Papovich et al. 2001; Ferguson et al. 2002; Papovich et al. 2005) that galaxies of…”