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Index - ... - The Optical Follow-up - The Radio Follow-up - The NIR Follow-up - ...

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High redshift radio galaxies play an important role in cosmology. It is likely that they are the oldest and most massive galaxies at high redshifts, and as such they place important constraints upon the epoch at which the first generation of stars was formed.

Furthermore, recent evidence has convincingly shown that powerful high redshift radio galaxies lie towards the centre of rich (proto-)clusters of galaxies. This evidence includes: (i) the detection of X-ray emission from the vicinity of a number of z ~ 1 3CR radio galaxies which has been associated with cooling flows of a few hundred M yr^-1 in relatively dense intracluster media ( Crawford and Fabian 1995 and references therein). Even a radio galaxy at z = 2.156 has been convincingly detected (Carilli et al. 1998). (ii) In narrow band [OII] imaging, companions are seen around many of the radio galaxies. (iii) The most conclusive evidence, however, comes from detailed studies of the fields around individual high redshift radio galaxies. Dickinson et al. have observed a number of z = 1 radio galaxies confirming tenth of associated clusters galaxies.

The proposed XMM survey will be of key importance for a number of important issues concerning the relation of (powerfull) radio galaxies and the overall mass distributions in the Universe:

(i) Radio sources as tracers of large scale structure

Recently, evidence for anisotropy in the distribution of radio sources has been presented and it has been shown that the difference in amplitude of the angular two point correlation function for various big modern radio catalogues (NVSS, FIRST, WENSS) must be related to differences in the intrinsic characteristics of the radio source populations probed by the radio surveys (Rengelink 1999). The interpretation of the angular correlation functions in the aforementioned surveys is hampered by a poor knowledge of the environment within which the radio sources are located. The proposed survey in combination with the new radio surveys will provide this.

(ii) The environment of radio sources and the evolution of the RLF

Hill & Lilly (1991) have shown that FRII radio appear in denser environments at z = 0.5 than at z = 0, while the environment of FRI radio sources remains the same. These data are compatible with a model of strong, luminosity dependent evolution in the environment of FRII radio sources, combined with little or no evolution in the environment of FRI sources. Assuming that sources which are tracers of denser environments also have a larger correlation length, an essential prediction is that the more luminous sources at high redshift should reside in relatively dense environments. This prediction is given a physical justification, and shown to be consistent with current observations, by Best, Longair & Röttgering (1997). However this model should be observationally tested by studying the environments of low luminosity radio sources out to z ~ 1. The proposed XMM survey is ideal for this.

(iii) Distant Radio Halos

Radio halos are rare regions of smooth extended radio emitting plasma with size of order 1 Mpc. Only 10 are known and all these sources are located in clusters with extreme properties (Hanish 1982): (i) they are among the richest clusters, (ii) they have an unusually low content of spiral galaxies (~ 10%), (iii) their galaxies have a large velocity dispersion (~ 1000 km/s), (iv) they have large X-ray luminosities (L ~ 10⁴⁴ - 10⁴⁵ erg/s) and large X-ray core radii (> 0.3 Mpc) and (v) they do not posses a cooling flow.