Galaxy clusters and large scale structure at high redshifts

We present a detailed study of a rich galaxy cluster at z=2.38. We demonstrate that this cluster contains large overdensities of damped Ly alpha absorption lines, of Ly alpha emitting galaxies and of extremely red objects. The overdensity of extremely red objects in this field demonstrates that many are high z galaxies. The huge overdensities we measure for these three classes of object are much larger than the mass overdensities of typical clusters at this redshift, as predicted by CDM and related models. We suggest therefore that the distribution of damped Ly alpha absorption line systems, of Ly alpha emitting galaxies and of extremely red objects are all very strongly biassed, and that somehow a small overdensity of mass has increased the fraction of baryons in collapsed objects, in the volume occupied by the cluster, to close to unity (a factor of similar to 10 increase). We speculate that some unknown physical process, acting on the volume occupied by our cluster, caused the normally diffuse ionised intergalactic medium to coalesce into small (< 10(8) M.) blobs of neutral hydrogen, which produce the Ly alpha absorption-lines. Star formation occurred within these blobs at z > 5, enriching them with metals and producing stars, which after several mergers and similar to 0.5 Gyr of passive evolution form the extremely red objects. The Ly alpha emitting galaxies are probably AGN, triggered perhaps by mergers of the small blobs.