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, starbursts, and other dramatic changes in morphology and luminosity. The measurement of the evolution of the mass function of galaxies is therefore essential. This can be accomplished by measuring the evolution of the mass-to-light ratios of galaxies as a function of redshift. The Fundamental Plane relation 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 cosmology 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 early-type galaxies in clusters follow a well-defined relation out the highest redshift. The mass-to-light evolution is very slow, and implies a high mean stellar age in an open universe.