Se p 20 00 Formation and Evolution of E and S 0 Galaxies from HST and Keck Studies of z ∼ 0 . 3 – 1 Clusters

We have witnessed a dramatic increase over the last five years in results on distant galaxies, in large part because of the high resolution imaging capability of HST, and the multiobject spectroscopic capability of the Keck telescopes. Our program to obtain wide-field, multi-color WFPC2 mosaics with HST of intermediate redshift clusters, and spectroscopic membership and high S/N spectroscopy with LRIS on Keck, has provided new insights into the nature of elliptical and S0 galaxies in the cluster environment over a wide range of densities. In particular, most ellipticals, and a significant fraction of the S0 population, have large luminosity-weighted ages, suggesting that their stellar populations were formed at redshifts beyond z ∼ 2, though the existence of substantial numbers of major mergers in MS 1054-03 at z = 0.83 suggests that final assembly of such galaxies may not have occurred until much later for a significant fraction of early-type galaxies.