Protoclusters associated with distant radio galaxies

The results of a large program conducted with the Very Large Telescope and the Keck telescope to search for forming clusters of galaxies near powerful radio galaxies at 2 < z < 5.2 are presented. Deep narrow- and broad-band images of eight radio galaxies and their surroundings were obtained. In the images a total of 335 candidate Ly alpha emitting galaxies at the redshift of the radio galaxies were selected. Subsequently, follow-up spectroscopy was performed to obtain redshifts for the candidate Ly alpha emitters. In at least six of eight fields (75%) an overdensity of Ly alpha emitters is found near the radio galaxy. Taken together with a significant clustering in redshift space of the emitters, we argue that we have discovered six forming clusters of galaxies (protoclusters). Based on the estimated masses and the volume density, we conclude that the observed structures are the likely progenitors of present-day massive (similar to 10(15) M circle dot) clusters of galaxies. Finally, we compare the observed galaxy overdensities with numerical simulations of the development of large scale structure in the Universe. The preliminary results of this comparison provide strong additional evidence for the hypothesis that the radio galaxy protoclusters are forming massive clusters.