The Epochs of Early Type Galaxy Formation as a Function of Environment

  • Thomas D
  • Maraston C
  • Bender R
 et al. 
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Abstract

The aim of this paper is to set constraints on the epochs of early-type
galaxy formation through the ``archaeology'' of the stellar populations
in local galaxies. Using our models of absorption-line indices that
account for variable abundance ratios, we derive ages, total
metallicities, and element ratios of 124 early-type galaxies in high-
and low-density environments. The data are analyzed by comparison with
mock galaxy samples created through Monte Carlo simulations taking the
typical average observational errors into account, in order to eliminate
artifacts caused by correlated errors. We find that all three
parameters, age, metallicity, and α/Fe ratio, are correlated with
velocity dispersion. We show that these results are robust against
recent revisions of the local abundance pattern at high metallicities.
To recover the observed scatter we need to assume an intrinsic scatter
of about 20% in age, 0.08 dex in [Z/H], and 0.05 dex in [α/Fe].
All low-mass objects with
M*s-1) show evidence for the presence of intermediate-age
stellar populations with low α/Fe ratios. About 20% of the
intermediate-mass objects with
1010[110lenticular galaxies] must have either a young subpopulation or a blue
horizontal branch. On the basis of the above relationships, valid for
the bulk of the sample, we show that the Mg-σ relation is mainly
driven by metallicity, with similar contributions from the α/Fe
ratio (23%) and age (17%). We further find evidence for an influence of
the environment on the stellar population properties. Massive early-type
galaxies in low-density environments seem on average ~2 Gyr younger and
slightly (~0.05-0.1 dex) more metal-rich than their counterparts in
high-density environments. No offsets in the α/Fe ratios are
instead detected. With the aid of a simple chemical evolution model, we
translate the derived ages and α/Fe ratios into star formation
histories. We show that most star formation activity in early-type
galaxies is expected to have happened between redshifts ~3 and 5 in
high-density environments and between redshifts 1 and 2 in low-density
environments. We conclude that at least 50% of the total stellar mass
density must have already formed at z~1, in good agreement with
observational estimates of the total stellar mass density as a function
of redshift. Our results suggest that significant mass growth in the
early-type galaxy population below z~1 must be restricted to less
massive objects, and a significant increase of the stellar mass density
between redshifts 1 and 2 should be present, caused mainly by the field
galaxy population. The results of this paper further imply the presence
of vigorous star formation episodes in massive objects at z~2-5 and
evolved elliptical galaxies around z~1, both observationally identified
as SCUBA galaxies and extremely red objects, respectively.

Author-supplied keywords

  • Galaxies: Abundances
  • Galaxies: Elliptical and Lenticular
  • Galaxies: Evolution
  • Galaxies: Formation
  • Galaxies: Stellar Content
  • cD

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Authors

  • Daniel Thomas

  • Claudia Maraston

  • Ralf Bender

  • Claudia Mendes de Oliveira

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