Clustering and Proto-Clusters in the Early Universe

The clustering properties of clusters, galaxies and AGN as a function of redshift are briefly discussed. It appears that extremely red objects at z ∼ 1, and objects with J − K > 1.7 and photometric redshifts 2 < zphot < 4 are highly clustered, indicating that a majority of these objects constitutes the progenitors of nearby ellipticals. Similarly clustered seem luminous radio galaxies at z ∼ 1, indicating that these objects comprise a short lived phase in the lifetime of these red objects. The high level of clustering furthermore suggests that distant powerful radio galaxies (e.g. z > 2) might be residing in the progenitors of nearby clusters – proto-clusters. A number of observational projects targetting fields with distant radio galaxies, including studies of Lyα and Hα emitters, Lyman break galaxies and (sub)mm and X-ray emitters, all confirm that such radio galaxies are indeed located in proto-clusters. Estimates of the total mass of the proto-clusters are similar to the masses of local clusters. If the total star formation rate which we estimate for the entire proto-clusters is sustained up to z ∼ 1, the metals in the hot cluster gas of local clusters can easily be accounted for.