The evolution and merging history of cluster ellipticals from z=0 to z=0.83

The evolution of galaxies is likely to be complex, involving mergers, starb ursts, and other dramatic changes in morphology and luminosity. The measure ment of the evolution of the mass function of galaxies is therefore essenti al. This can be accomplished by measuring the evolution of the mass-to-ligh t ratios of galaxies as a function of redshift. The Fundamental Plane relat ion is uniquely suited to measure the evolution of the mass-to-light ratio of early-type galaxies. We show that the evolution depends sensitively on c osmology and star-formation history. We present results on the evolution of the mass-to-light ratio from the Fundamental Plane out to z = 0.83. The ea rly-type galaxies in clusters follow a well-defined relation out the highes t redshift. The mass-to-light evolution is very slow, and implies a high me an stellar age in an open universe. One of the main uncertainties in the interpretation is morphological evolut ion. If the youngest early types at low redshift appear as other morphologi cal types at high redshift, then the study of early-type galaxies at high r edshifts will produce biased results. We discuss the effects of this 'proge nitor bias'. We show evidence for significant morphological evolution for a ll early types (elliptical and S0 galaxies). We find a high fraction of mer gers in MS 1054-03, comparable with the fraction of ellipticals. Furthermor e, the total fraction of early types in rich clusters decreases from z = 0 to z = 0.83. These results suggest that the set of early types is not a clo sed set, but evolving. The effects on the derived evolution of the mass-to- light ratio is relatively small, due to the small scatter. The next step will be to extend these studies to high redshift; clusters, a nd to the field. This work can provide very strong constraints on the mass evolution of galaxies.