Clustering around radio galaxies at z similar to 1.5

The importance of studying old elliptical galaxies at redshift z similar to 1.5 is reviewed, considering both what can be learned by extending studies of the evolution of cluster galaxy scaling relations to earlier cosmic epochs, and the age-dating of old elliptical galaxies at high redshifts. Following this, the first results are provided of an on-going project to find such distant elliptical galaxies, through an investigation of the cluster environments of powerful radio sources with redshifts 1.44 < z < 1.7. These studies show a considerable excess of red galaxies in the radio source fields, with the magnitudes (K greater than or similar to 17.5) and colours (R - K > 4) expected of old passively evolving galaxies at the radio source redshift. The red galaxy overdensities are found on two different scales around the radio sources; a pronounced small-scale peak at radial distances of less than or similar to 150 kpc, and a weaker large-scale excess extending out to 1-1.5 Mpc. The presence and richness of these red galaxy excesses varies considerably from source-to-source. An interpretation of these results is provided. (C) 2003 Elsevier B.V. All rights reserved.