Evolution of Powerful AGN and the Collapse of Rich Clusters

Abstract

The cosmological evolution of active galactic nuclei (AGN) may be described by a luminosity function which depends on redshift and on the (stellar) luminosity or mass of the galaxy. Our mathematical expression for the luminosity function of AGN is based on the “loss-cone filling” model for the fuelling of central black holes in AGN during galaxy mergers. We assume that galactic nuclei containing massive central black holes were formed at z ≃ 3 and that the subsequent cosmological evolution of AGN is mainly due to a decrease in the merging rate among galaxies during the subsequent epoch of cluster formation. The radio (and optical) counts of AGN can be reproduced if the merging rate of the most massive galaxies evolves faster than that of the smaller galaxies. At redshifts of about 2.5 the evolution is very similar to pure luminosity evolution and the space density of the most luminous AGN is comparable to the space density of the most massive galaxies. The rapid evolution of powerful AGN is interpreted as an evolution of the merging rate of the brightest galaxies during the collapse and virialization of rich clusters.