Evolution and Environment of Early-Type Galaxies

By Mariangela Bernardi, Robert Nichol, R. Sheth, C. J. Miller, J. Brinkmann. 2005

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Chemical abundance indicators are studied using composite spectra, which we provide in tabular form. Tables of line strengths measured from these spectra and parameters derived from these line strengths are also provided. From these we find that at fixed luminosity, early-type galaxies in low-density environments are slightly bluer, with stronger O II emission and stronger Hδ and Hγ Balmer absorption lines, indicative of star formation in the not very distant past. These galaxies also tend to have systematically weaker D4000 indices. The Lick indices and α-element abundance indicators correlate weakly but significantly with environment. For example, at fixed velocity dispersion, Mg is weaker in early-type galaxies in low-density environments by 30% of the rms scatter across the full sample, whereas most Fe indicators show no significant environmental dependence. The galaxies in our sample span a redshift range that corresponds to look-back times of ∼1 Gyr. We see clear evidence for evolution of line-index strengths over this time. Since the low-redshift population is almost certainly a passively aged version of the more distant population, age is likely the main driver for any observed evolution. We use the observed redshift evolution as a model-independent clock to identify indicators that are more sensitive to age than to other effects such as metallicity. In principle, for a passively evolving population, comparison of the trends with redshift and environment constrain how strongly the luminosity-weighted ages and metallicities depend on environment. We develop a method for doing this that does not depend on the details of stellar population synthesis models. Our analysis suggests that the galaxies that populate the densest regions in our sample are older by ∼1 Gyr than objects of the same luminosity in the least dense regions, and that metallicity differences are negligible. We also use single-burst stellar population synthesis models, which allow for nonsolar α-element abundance ratios, to interpret our data. The combination of Hβ, Mg b, and ⟨Fe⟩ lines suggests that age, metallicity, and α-enhancement all increase with velocity dispersion. The objects at lower redshifts are older but have the same metallicities and α-enhancements as their counterparts of the same σ at higher redshifts, as expected if the low-redshift sample is a passively aged version of the sample at higher redshifts. In addition, objects in dense environments are less than 1 Gyr older and α-enhanced by ∼0.02 relative to their counterparts of the same velocity dispersion in less dense regions, but the metallicities show no dependence on environment. This suggests that in dense regions, the stars in early-type galaxies formed at slightly earlier times and on a slightly shorter timescale than in less dense regions. Using HγF instead of Hβ leads to slightly younger ages but the same qualitative differences between environments. In particular, we find no evidence that objects in low-density regions are more metal-rich.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Galaxy bimodality versus stellar mass and environment (2006, background): “For the red sequence, this can be explained by a small difference in age between low- and high-density environments of ∼ 2Gyr (Thomas et al. 2005) or less (Hogg et al. 2004; Bernardi et al. 2006).”
The 2df SDSS LRG and QSO survey: evolution of the luminosity function of luminous red galaxies to z= 0.6 (2006, background): “…over a range of redshifts (Bernardi et al. 1998; Kuntschner & Davies 1998; Trager et al. 2000; Hogg et al. 2002; Bernardi et al. 2003a, 2003b,c,d; Baldry et al. 2004; Cimatti et al. 2004; Glazebrook et al. 2004; McCarthy et al. 2004; Papovich et al. 2005; Thomas et al. 2005; Bernardi et al. 2006).”
Stellar masses of SDSS-III/BOSS galaxies at z ∼ 0.5 and constraints to galaxy formation models (2012, background): “2011; De Lucia & Borgani 2012); the uniformly old stellar ages with little evidence of star formation (Bower et al. 1992, 1998; Thomas et al. 2005; Bernardi et al. 2006), the independence of the stellar population properties of the environment (Peng et al. 2010; Thomas et al. 2010).”
Stellar Population Diagnostics of Elliptical Galaxy Formation (2006, methodology): “For example, in a recent study of local (z ∼ 0) ETGs from the Sloan Digital Sky Survey (SDSS) all three criteria were adopted (Bernardi et al. 2006), and the result is reported in Table 1 (M. Bernardi, private communication).1 From a sample including ∼123,000 galaxies with 14.5 < rPetrosian < 17.77…”
The SAURON project – XVII. Stellar population analysis of the absorption line strength maps of 48 early-type galaxies (2010, background): “In agreement with previous investigations (e.g., Kuntschner et al. 2002; Thomas et al. 2005; Bernardi et al. 2006; Collobert et al. 2006; Thomas et al. 2010) there is mild evidence for early-type galaxies residing in low density regions to have slightly younger ages on average compared to the…”
Observational evidence for AGN feedback in early-type galaxies (2007, background): “…short timescales as suggested by the stellar population properties of nearbysamples (e.g. Trager et al. 2000; Thomas et al. 2005; Nelan et al.2005; Bernardi et al. 2006; Jimenez et al. 2006) as well as the redshift evolution of luminosity and mass functions of massive galaxies (e.g. Wake et al.…”
Physical properties and environments of nearby galaxies (2009, background): “Bernardi et al. (2006) found qualitatively similar results, though with a rather different analysis technique.”
Environmental Dependence of Properties of Galaxies in the Sloan Digital Sky Survey (2006, background): “The very slight dependence of the location of the red sequence on the density field has been noted in the past, originally by Sandage & Visvanathan (1978), but more recently by others (Balogh et al. 2004a; Hogg et al. 2004; Bernardi et al. 2006; Blanton et al. 2007).”
GALAXY ZOO: THE FUNDAMENTALLY DIFFERENT CO-EVOLUTION OF SUPERMASSIVE BLACK HOLES AND THEIR EARLY- AND LATE-TYPE HOST GALAXIES (2010, background): “…and by probing ongoing star formation at increasing redshifts (Cowie et al. 1996; Heavens et al. 2004; Thomas et al. 2005; Nelan et al. 2005; Bernardi et al. 2006; Bundy et al. 2006; Jimenez et al. 2007; Pérez-González et al. 2008; Collins et al. 2009; Thomas et al. 2009), and has…”
The ATLAS3D Project – XXX. Star formation histories and stellar population scaling relations of early-type galaxies (2015, background): “…galaxies appear older, metalrich, and with enhanced abundance ratios with respect to lower mass objects – a picture emphatically confirmed with very large samples of galaxies from modern surveys (Bernardi et al. 2006; Gallazzi et al. 2006; Graves, Faber & Schiavon 2009a; Thomas et al. 2010).”
Curvature in the scaling relations of early-type galaxies (2008, methodology): “At the high end we select galaxies with σ < 400 km s−1 to avoid contamination from double/multiple superpositions (see Bernardi et al. 2006b, 2008).”
Environment and self-regulation in galaxy formation (2009, methodology): “On top of this selection effect in σ , small evolutionary effects complicate the derivation of stellar population parameters in spite of the small time window sampled (Bernardi et al. 2006).”
Flux-calibrated stellar population models of Lick absorption-line indices with variable element abundance ratios (2010, background): “…instead, discloses stellar population properties that act as fossil record and can be used to derive ages, formation epochs, star formation histories, and element abundances (e.g. Trager et al. 2000a; Kuntschner 2000; Thomas et al. 2005; Nelan et al. 2005; Bernardi et al. 2006; Thomas et al. 2010).”
THE INITIAL MASS FUNCTION OF EARLY-TYPE GALAXIES (2010, unknown): “This appears to be quite a dramatic change over a factor of ∼ 2 in velocity dispersion, corresponding approximately to 0.15 dex change in [Fe/H] and 0.1 dex in [α/Fe] (Bernardi et al. 2006).”
Ages and metallicities of early-type galaxies in the SDSS: new insight into the physical origin of the colour-magnitude and the Mg2-sigmaV relations (2006, result): “Bernardi et al. (1998), later confirmed by Bernardi et al. (2006) on a larger sample of SDSS early-type galaxies, found differences in the Mg2−σV relation of galaxies in different environments, implying that galaxies in dense environments are at most 1 Gyr older than galaxies in low-density…”