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 a well defined magnitude limit (M_B\leq -20.0 in the field and M_B\leq -20.5 in the 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 similar in the field and in the 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 the age of early-type galaxies increases with galaxy mass (downsizing) in all environments, and that cluster galaxies appear to have the same age within 5% as field galaxies at any given galaxy mass. The first result confirms similar ones obtained by other means, while the second one is controversial. The most recent incarnation of the hierarchical models of galaxy formation and evolution is capable of explaining the first result, but predicts that cluster galaxies should be older than field galaxies. We also find a total lack of massive early--type galaxies M>3 10^{11} M_{\sun}) with a formation redshift smaller than 2, which cannot be due to selection effects. 
Sperello di Serego Alighieri, Barbara Lanzoni and Inger Jorgensen