The Evolution in the Faint-End Slope of the Quasar Luminosity Function

Using a new model for quasar lifetimes and light curves derived from numerical simulations of galaxy mergers that incorporate black hole growth, we study the faint-end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our model motivates a new interpretation of the QLF in which the bright end consists of quasars radiating near their peak luminosities but the faint end is mostly made up of brighter peak luminosity quasars seen in less luminous phases of evolution. The faint-end slope of the QLF is then set by the behavior of the lifetime (light curve) of quasars with peak luminosities near the observed break when they are in less luminous stages of evolution. We determine the faint-end slope of the QLF from the quasar lifetime, based on a set of simulations that encompass a wide range of host galaxy, merger, black hole, and interstellar gas properties. Brighter peak luminosity (higher black hole mass) systems undergo more violent evolution, and gas is expelled and heated more rapidly in the final stages of quasar evolution, resulting in a flatter faint-end slope (as these objects fall below the observed break in the QLF more rapidly). Therefore, as the QLF break luminosity moves to higher luminosities with increasing redshift, implying a larger typical quasar peak luminosity, the faint-end QLF slope flattens. From our model, we predict the evolution of the faint-end slope of the QLF and find good agreement with observations. Although black holes grow in an antihierarchical manner (with lower mass black holes formed primarily at lower redshifts), in our picture the observed change in slope and differential or "luminosity-dependent density evolution" in the QLF is determined by the nontrivial, luminosity-dependent quasar lifetime and physics of quasar feedback, and not by changes in the shape of the underlying peak luminosity or active black hole mass distributions.