The Impact of Energy Feedback on Quasar Evolution and Black Hole Demographics

Abstract

We investigate how accretion episodes onto massive black holes power quasars and AGN while they accumulate mass into the holes. We compute both the trend and the stochastic component to the trigger of the accretion events, as provided by structure buildup after the hierarchical paradigm. We base on host galaxy evolution proceeding from the protogalactic era at redshifts z > 2.5 dominated by major merging events in high density regions, to the subsequent era marked by galaxy-galaxy interactions in newly forming groups. These dynamical events perturb the gravitational equilibrium of the gas reservoir in the hosts, and trigger recurrent accretion episodes controlled by energy feedback from the very source emission onto the surrounding gas. Depletion of the latter by these events (adding to quiescent star formation) concurs with the slowing down of the clustering to cause a fast drop of the activity in dense regions. Meanwhile, in the "field" later and rarer events are triggered by interactions of still gas-rich galaxies, and eventually by captures of dwarf satellite galaxies; these are also included in our analytic model. Thus we compute the quasar and AGN luminosity functions; we find these to brighten and rise from z ~ 6 to z ~ 2.5, and then toward z ~ 0 to dim and fall somewhat, in detailed agreement with the observations. We predict that for z < 2.5 the mass distribution of the holes progressively rises and shifts rightwards; we compare our results with the local data. We also find that downward of z ~ 2.5 the Eddington ratios related to emitting, most massive holes drift below unity on average, with a widening scatter; meanwhile, some smaller holes flare up closer to the Eddington limit.