The 3CR radio galaxies at z = 1: Old stellar populations in central cluster galaxies

We investigate the old stellar populations of the 3CR radio galaxies at redshift z ~ 1, using observations made with the Hubble Space Telescope and the United Kingdom InfraRed Telescope. At radii r < 35 kpc, the infrared radial intensity profiles of the galaxies follow de Vaucouleurs' law, whilst at larger radii the galaxies show an excess of emission similar to that of low redshift cD galaxies. The locus of the high redshift 3CR galaxies on the Kormendy relation is investigated: passive evolution of the stellar populations is required to account for their offset from the relation defined by low redshift giant ellipticals and brightest cluster galaxies. The 3CR galaxies, on average, possess larger characteristic radii than the low redshift brightest cluster galaxies. Coupled with existing evidence, these results are strongly suggestive that distant 3CR galaxies must be highly evolved systems, even at a redshift of one, and lie at the centre of moderate to rich (proto-)clusters.