The Influence of Mass and Environment on the Evolution of Early-Type Galaxies

We report on a uniform comparative analysis of the fundamental parameters of early-type galaxies at z~1 down to M_B<=-20, both in the field and in clusters. The changes in the M/L_B ratio from z~1 to today are larger for lower mass galaxies in all environments, and are larger in the field than in clusters for galaxies with the same mass. By deriving ages from the M/L_B ratio, we estimate the formation redshift for early-type galaxies as a function of galaxy mass and environment. We find that (1) the age of early-type galaxies increases with galaxy mass (downsizing) in all environments, (2) cluster galaxies are older than field galaxies at any given galaxy mass, and (3) this age difference increases with galaxy mass. The first two results confirm similar ones obtained by other means, while the third one is new. The most recent incarnation of the hierarchical models of galaxy formation and evolution is capable of explaining the first two results, but predicts the opposite of our third result. We also find a total lack of massive early-type galaxies (M>3×10¹¹ M_solar) with a formation redshift smaller than 2, which cannot be due to selection effects.