Measuring the evolution of the mass-to-light ratio from z=0 to z=0.6 from the fundamental plane

Galaxy evolution is probably a complex process. Mergers, infall, and starbursts may change galaxy properties systematically with time. As a result, the interpretation of the luminosity function is ambiguous, and information on the mass evolution of galaxies is needed. Such information can be retrieved from the evolution of the Tully-Fisher relation, Faber-Jackson relation, or the Fundamental Plane with redshift. Observations of this kind have recently become possible. We present the Fundamental Plane relation measured for galaxies in the rich clusters out to z = 0.58. The galaxies satisfy a tight Fundamental Plane, with relatively low scatter (17%). The M/L ratio evolves slowly with redshift, In M/L-V proportional to 0.8z. This result is consistent with simple evolutionary models if the bulk of the stellar population formed at high redshift. It is not clear yet how these results can be made consistent with the rapid evolution of galaxies in intermediate redshift clusters as indicated by the Butcher-Oemler effect. Observations of post-starburst galaxies ("E+A" galaxies) indicate that these systems are dominated by disks. They may evolve into galaxies which are underrepresented in most "normal" Fundamental Plane samples.