The high redshift 3CR radio galaxies: cD galaxies in high redshift clusters

The old stellar populations of the 3CR radio galaxies at redshifts z similar to 1 have been investigated using the HST and UKIRT. The radial intensity profiles of the galaxies are well matched out to radii r similar to 35 kpc by de Vaucouleurs' law with no requirement for an additional point source in all but two cases (3C22 and 3C41). At larger radii the galaxies show an excess of emission similar to that of the halos seen around cD galaxies. The high redshift 3CR galaxies have larger characteristic radii, on average, than low redshift brightest cluster galaxies, indicating that they must be highly evolved dynamically, even at a redshift of one. This supports other independent evidence for the hypothesis that these galaxies lie in cluster environments. The difference between the K-z relationships of the 3CR and 6C galaxies is investigated: the 3CR galaxies must contain a greater mass of stars, consistent with them lying at the centre of clusters. The consequences of these results are discussed.