THE EVOLUTIONARY STATUS OF CLUSTERS OF GALAXIES AT Z ∼ 1

Abstract Combined HST, X-ray, and ground-based optical studies show that clusters of galaxies are largely "in place " by z ∼ 1, an epoch when the Universe was less than half its present age. High resolution images show that elliptical, S0, and spiral galaxies are present in clusters at redshifts up to z ∼ 1.3. Analysis of the CMDs suggest that the cluster ellipticals formed their stars several Gyr earlier, at z ∼> 3. The morphology–density relation is well established at z ∼ 1, with star-forming spirals and irregulars residing mostly in the outer parts of the clusters and E/S0s concentrated in dense clumps. The intracluster medium has already reached the metallicity of present-day clusters. The distributions of the hot gas and early-type galaxies are similar in z ∼ 1 clusters, indicating both have largely virialized in the deepest potentials wells. In spite of the many similarities between z ∼ 1 and present-day clusters, there are significant differences. The morphologies revealed by the hot gas, and particularly the early-type galaxies, are elongated rather than spherical. We appear to be observing the clusters at an epoch when the sub-clusters and groups are still assembling into a single regular cluster. Support for this picture comes from CL0152 where the gas appears to be lagging behind the luminous and dark mass in two merging sub-components. Moreover, the luminosity difference between the first and second brightest cluster galaxies at z ∼ 1 is smaller than in 93 % of present-day Abell clusters, which suggests that considerable luminosity evolution through merging has occurred since that epoch. Evolution is also seen in the bolometric X-ray luminosity function