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

Abstract

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. One of the main uncertainties in the interpretation is morphological evolution. If the youngest early types at low redshift appear as other morphological types at high redshift, then the study of early–type galaxies at high redshifts will produce biased results. We discuss the effects of this ‘progenitor bias’. We show evidence for significant morphological evolution for all early types (elliptical and S0 galaxies). We find a high fraction of mergers in MS 1054–03, comparable with the fraction of ellipticals. Furthermore, 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 closed 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, and to the field. This work can provide very strong constraints on the mass evolution of galaxies.