The cluster environments of the z~1 3CR radio galaxies

Abstract

An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6<z<1.8 is presented, based primarily upon K-band images down to K~20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K-band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular cross-correlation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters. These data are complemented by J-band images also from UKIRT, and by optical images from the Hubble Space Telescope. The fields of the lower redshift (z<~0.9) radio galaxies in the sample generally show well-defined near-infrared colour-magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000Å break show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or on-going star formation. At higher redshifts the colour-magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very red infrared colours (J-K>~1.75) expected of old cluster galaxies at these redshifts. Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z~1 radio galaxies lie in (proto)cluster environments.