Clustering of red and blue galaxies around high-redshift 3C radio
  sources as seen by the Hubble Space Telescope
  To properly understand the evolution of high-redshift galaxy clusters, both
passive and star-forming galaxies have to be considered. Here we study the
clustering environment of 21 radio galaxies and quasars at 1<z<2.5 from the
third Cambridge catalog (3C). We use optical and near-infrared Hubble Space
Telescope images with a 2' field-of-view, where the filters encompass the
rest-frame 4000 Angstroem 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. We find that about 16 of 21 radio sources
inhabit a galaxy overdensity on scales of 250 kpc (30") 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 z >
1.6, red overdensities at 1.2<z<1.6, and red overdensities with an increased
deficit of central blue galaxies at z<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 3C clusters are already mass-rich and compact.
