Quasar evolution driven by galaxy encounters in hierarchical structures

We link the evolution of the galaxies in the hierarchical clustering scenario with the changing accretion rates of cold gas onto the central massive black holes that power the quasars. We focus on galaxy interactions as main triggers of accretion; the related scaling laws are taken from Cavaliere & Vittorini and grafted to a semianalytic code for galaxy formation. As a result, at high z the protogalaxies grow rapidly by hierarchical merging; meanwhile, much fresh gas is imported and also destabilized, so the holes are fueled at their full Eddington rates. At lower z the galactic dynamical events are mostly encounters in hierarchically growing groups; now the refueling peters out, as the residual gas is exhausted while the destabilizing encounters dwindle. So, with no parameter tuning other than that needed for stellar observables, our model uniquely produces a rise from z approximate to 6 to 2.5, and for z < 2.5 a decline of the bright quasar population as steep as observed. In addition, our results closely fit the observed luminosity functions of quasars, their space density at different magnitudes from to z approximate to 5 to 0, and the local m(BH)-sigma relation.