The Epochs of Early-Type Galaxy Formation in Clusters and in the Field

Abstract

Using our models of absorption line indices that account for variable abundance ratios, we derive ages, total metallicities, and element abundance ratios of 126 early-type galaxies in various 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 further find evidence for an influence of the environment on the stellar population properties. Massive early-type galaxies in low-density environments appear on average several Gyrs younger and ~ 0.1 dex more metal-rich than their counterparts in clusters. No offsets in the α/Fe ratios, instead, are 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 and between redshifts 2 and 3 in low density environments.