Formation and Evolution of
Galaxy Clusters Across Cosmic Time

A sample of 21 radio galaxies and quasars at redshift greater than 1.0 and less than 2.5 from the third Cambridge catalog (3C) has been mapped with the Hubble Space Telescope (HST) in two filters encompassing the rest frame 4000 Å break. Passive red and star-forming blue galaxies were separated in the color–magnitude diagram using a redshift-dependent cut derived from galaxy evolution models. Within 2′ field-of-view, 16 of 21 radio sources inhabit a galaxy overdensity on a scale of 30′′ (250 kpc) projected radius. The sample shows a diversity of red and blue overdensities and also sometimes a deficiency of blue galaxies in the center. The following tentative evolutionary trends are seen: extended proto-clusters with only weak overdensities at redshift greater than 1.6, red overdensities at redshift greater than 1.2 and less than 1.6, and red overdensities with an increased deficit of central blue galaxies at redshift less than 1.2. Only a few 3C sources show a blue overdensity tracing active star-formation in the cluster centers; this rarity could indicate that the powerful quasar activity may quench star-formation in the vicinity of most radio sources. The derived number of central luminous red galaxies and the radial density profiles are comparable to those found in local clusters, indicating that some high-z 3C clusters are already mass-rich and compact.