Evolution of early-type galaxies in clusters

The slow evolution of the M/L ratios, colors, and line strengths of cluster early-type galaxies to z ∼ 1 suggests that their stars were formed at very high redshift. At the same time, morphological studies of distant clusters indicate significant evolution in the early-type galaxy population. Striking evidence for strong morphological evolution in clusters is the discovery of a large number of red merger systems in the cluster MS 1054–03 at z = 0.83. The presence of these mergers is qualitatively consistent with predictions from hierarchical galaxy formation models, and is direct evidence against an early collapse for all early-type galaxies. In most of the mergers there is no evidence for strong star formation. Therefore the mean stellar ages of the merger products will be much older than the “assembly age”, and do not violate the constraints on the star formation epoch of early-type galaxies imposed by the color-magnitude relation and the Fundamental Plane.