Proto-Clusters with Evolved Populations around Radio Galaxies at z~2.5

Abstract

We report a discovery of proto-cluster candidates around high redshift radio galaxies at z~2.5 on the basis of clear statistical excess of colour-selected galaxies around them seen in the deep near-infrared imaging data obtained with CISCO on Subaru Telescope. We have observed six targets, all at similar redshifts at z~2.5, and our data reach to J=23.5, H=22.6 and K=21.8 (5 sigma) and cover a 1.6&#39; x 1.6&#39; field centered on each radio galaxy. We apply colour cuts in JHKs in order to exclusively search for galaxies located at high redshifts, z>2. Over the magnitude range of 19.5<K<21.5 we see a significant excess of red galaxies with J-K>2.3 by a factor of two around the combined radio galaxies fields compared to those found in the general field of GOODS South. The excess of galaxies around the radio galaxies fields becomes more than factor of three around 19.5<K<20.5 when the two-colours cuts are applied with JHKs. Such overdensity of the colour-selected galaxies suggest that those fields tend to host high density regions at high redshifts, although there seems to be the variety of the density of the colour-selected galaxies in each field. In particular, two radio galaxies fields out of the six observed fields show very strong density excess and these are likely to be proto-clusters associated to the radio galaxies which would evolve into rich clusters of galaxies dominated by old passively evolving galaxies.