The Fall of the Quasar Population


We derive quantitative predictions of the optical and X-ray luminosity functions for quasars in the redshift range z ∼< 3. Based on accreting black holes as primary sources for the quasar outputs, we investigate how the accretion is controlled by the surrounding structures, as these grow hierarchically from the formation of the host galaxies to their assemblage into poor and eventually into rich groups. We argue that for z < 3 efficient black hole fueling is triggered by the encounters of a gas-rich host with its companions in a group; these destabilize the gas and induce accretion, giving rise to the following features. The dispersion of the dynamical parameters in the encounters produces luminosity functions with the shape of a double power-law. Strong luminosity evolution is produced as these encounters deplete the gas supply in the host; an additional, milder density evolution obtains since the interactions become progressively rarer as the groups grow richer but less dense. We carry out these arguments to derive a specific model for the evolving luminosity functions. From the agreement with the optical and the X-ray data, we conclude that the evolution of the bright quasars is driven by the development of cosmic structures in two ways. Earlier than z ∼ 3 the gas-rich protogalaxies grow by merging, which also induces parallel growth of central holes accreting at their full Eddington rates. In the later era of group assemblage the host encounters with companions drive onto already existing holes further but meager accretion; these consume the gas reservoirs in the hosts, and cause supply-limited emissions which are intermittent, go progressively sub-Eddington and peter out. Then other fueling processes occurring in the field come to the foreground; we specifically discuss the faint emissions, especially noticeable in X-rays, which are expected when hosts in the field cannibalize satellite galaxies with their meager gas contents. Subject headings: Quasars: general – galaxies: nuclei – galaxies: interactions – galaxies: clusters: general – X-rays: galaxies

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@inproceedings{Cavaliere2008TheFO, title={The Fall of the Quasar Population}, author={Alfonso Cavaliere and Vincenzo Vittorini}, year={2008} }