The Environment of z >1 3CR Radio Galaxies and QSOs: From Proto-Clusters
  to Clusters of Galaxies?
  We study the cluster environment for a sample of 21 radio loud AGN from the
3CR catalog at z>1, 12 radio galaxies and 9 quasars with HST images in the
optical and IR. We use two different approaches to determine cluster
candidates. We identify the early type galaxies (ETGs) in every field by
modeling each of the sources within a 40" radius of the targets with a Sersic
profile. Using a simple passive evolution model, we derive the expected
location of the ETGs on the red sequence (RS) in the color-magnitude diagram
for each of the fields of our sources. For seven targets, the model coincides
with the position of the ETGs. A second approach involves a search for over
densities. We compare the object densities of the sample as a whole and
individually against control fields taken from the GOODS-S region of 3D-HST
survey. With this method we determine the fields of 10 targets to be cluster
candidates. Four cluster candidates are found by both methods. The two methods
disagree in some cases, depending on the specific properties of each field. For
the most distant radio galaxy in the 3CR catalog (3C257 at z = 2.47), we
identify a population of bluer ETGs that lie on the expected location of the RS
model for that redshift. This appears to be the general behavior of ETGs in our
fields and it is possibly a signature of the evolution of such galaxies. Our
results are consistent with half of the z > 1 radio galaxies being located in
dense, rapidly evolving environments.
