Quasars and galaxy formation

Quasars are widely believed to be powered by accretion on to supermassive black holes and there is now considerable evidence for a link between mergers, the activity of quasars and the formation of spheroids. Cattaneo, Haehnelt & Rees have demonstrated that a very simple model in which supermassive black holes form and accrete most of their mass in mergers of galaxies of comparable masses can reproduce the observed relation of black hole mass to bulge luminosity if black holes accrete a fraction of the mass in the merging remnant that varies with redshift. Here we investigate whether this simple model can account for the luminosity function of quasars and for the redshift evolution of the quasar population. We simulate hierarchical galaxy formation through the extended Press–Schechter formalism and assume that, when two galaxies of comparable masses merge, their central black holes coalesce and a fraction of the gas in the merger remnant is accreted by the supermassive black hole. We find that the decrease in the merging rate with cosmic time and the depletion in the amount of cold gas available due to star formation are not sufficient to explain the strong decline in the space density of bright quasars since z∼2, if the fraction of accreted gas does not decrease at low redshift, as larger and larger structures form, which can potentially host brighter and brighter quasars. Moreover, we need another mechanism besides major mergers to explain the emission from the brightest quasars, and we speculate that fuelling from hot gas may be this mechanism.