The history of star formation and mass assembly in early-type galaxies

We define a volume-limited sample of over 14000 early-type galaxies (ETGs) selected from Data Release 6 of the Sloan Digital Sky Survey. The density of environment of each galaxy is robustly measured. By comparing narrow-band spectral line indices with recent models of simple stellar populations (SSPs), we investigate trends in the star formation history as a function of galaxy mass (velocity dispersion), density of environment and galactic radius. We find that age, metallicity and α-enhancement all increase with galaxy mass and that field ETGs are younger than their cluster counterparts by ~2Gyr. We find negative radial metallicity gradients for all masses and environments, and positive radial age gradients for ETGs with velocity dispersion over 180kms-1. Our results are qualitatively consistent with a relatively simple picture for ETG evolution in which the low-mass haloes accreted by a protoETG contained not only gas but also a stellar population. This fossil population is preferentially found at large radii in massive ETGs because the stellar accretions were dissipationless. We estimate that the typical, massive ETG should have been assembled at z <~ 3.5. The process is similar in the cluster and field but occurred earlier in dense environments.