The impact of energy feedback on quasar evolution and black hole demographics

We investigate how accretion episodes on to massive black holes power quasars and active galactic nuclei (AGNs) while they accumulate mass into the holes. We implement an analytic approach to 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 our approach on host-galaxy evolution proceeding from the protogalactic era at redshifts z greater than or similar to 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, first in the Eddington-limited regime, and later in a supply-limited mode controlled by energy feedback from the very source emission on to 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 similar or equal to 6 to z similar or equal to 2.5, and then towards z similar or equal to 0 to dim and fall somewhat, in detailed agreement with the observations. From the same accretion history 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 similar or equal to 2.5 the Eddington ratios related to emitting, most massive holes drift below unity on average, with a widening scatter; meanwhile, some smaller holes still flare up closer to the Eddington limit. We conclude that the accretion history, however rich, is dominated by dwindling events triggered by interactions under control by feedback; these establish a link between the declining but widely scattered distribution of the Eddington ratios and the tight, closely stable upper section of the M-sigma correlation. Two clear-cut predictions arise from our interaction picture: the black hole activity will appear to follow an anti-hierarchical trend; the quasar-AGN population is expected to be bimodal, and related to the bimodal galaxy population.