Cosmic Evolution of Active Galactic Nuclei

The X-ray sky is dominated by a diffuse extragalactic background radiation, which our team, together with others, was able to resolve almost completely into discrete sources using the X-ray satellites ROSAT, Chandra and XMM-Newton - we observe the growth phase of the population of supermassive black holes throughout the history of the Universe. Indeed, the mass distribution of black holes in local galaxies is well traced by the evolution of the accreting black hole luminosity function. However, the maximum of high-luminosity objects occurs significantly earlier in the history of the universe (at redshifts 2-3), than that of low-luminosity objects, which have a peak at redshifts below unity. This anti-hierarchical evolution is similar to the down-sizing effect observed in the optical galaxy population and requires different feeding modes in the early and late Universe. Galaxy mergers are likely responsible for the early growth of black holes and bulges, while a different feeding mode - the re-juvenation of black holes in galaxies by the accretion of fresh gas from their environment may be responsible for the late evolution of both the star formation and AGN activity. This is also confirmed by special correlation analyses, showing that AGN up to redshifts of z=2 live in relatively massive, non-evolving dark matter haloes.