Proto-clusters and the clustering of distant galaxies and radio sources

The clustering properties of objects in three different radio surveys (NVSS, FIRST and BOOTES-WSRT) and two near-infrared surveys (the 'Daddi field' and the FIRES survey) are investigated and compared with studies of various samples of galaxies, AGN and clusters. At z similar to 1, it seems that the 2dF optical quasars have a correlation length a factor of about 2 less than powerful radio galaxies at similar redshifts. This indicates that these two classes of object cannot be 'unified' by postulating that their main difference is due to their evolution being at a different stage. It seems much more likely that these QSOs are predominantly located in field galaxies, while the powerful radio sources are located preferentially in early types. Furthermore, it appears that both the extremely red objects (EROs) from the IR surveys and the more luminous radio sources are similarly clustered and as such are the most clustered objects known in the z > 1 Universe. Even at z similar to 3 the red J - K galaxies from the FIRES survey are similarly strongly clustered, at a level of about three times higher than Lyman break galaxies. These clustering properties are consistent with EROs and radio galaxies being similar objects at different stages of their evolution. Locally, the most clustered population of objects are clusters of galaxies. Since the progenitors of these objects-proto-clusters-will therefore also be highly clustered, a good way to locate proto-clusters is to target fields with very powerful and potentially highly clustered distant z > 2 radio sources. The techniques that are currently being used for locating and studying these proto-clusters are briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.