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

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 * ≲ 10 10 M ☉ (σ ≲ 130 km 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 10 10 ≲ M * / M ☉ ≲ 10 11 [110 ≲ σ/(km s -1 ) ≲ 230; both elliptical and lenticular 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.