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

Based on numerical simulations of galaxy mergers that incorporate black hole growth, we predict the faint end slope of the quasar luminosity function (QLF) and its evolution with redshift. Our simulations have yielded a new model for quasar lifetimes and light curves where the lifetime depends on both the instantaneous and peak luminosities of the quasar. This description motivates a new interpretation of the quasar luminosity function in which the bright end of the QLF 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 QLF slope is then determined by the faint-end slope of the quasar lifetime for quasars with peak luminosities near the observed break. We determine this slope from the quasar lifetime as a function of peak luminosity, based on a large set of simulations which encompass a wide variety of host galaxy, merger, black hole, and interstellar gas properties. Brighter peak luminosity (higher black hole mass) systems undergo more violent evolution, and expel and heat gas 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 determined quasar lifetimes as a function of peak luminosity and this interpretation of the QLF, we predict the faint-end QLF slope and its evolution with redshift in good agreement with observations. Although black holes grow in an anti-hierarchical manner (with lower-mass black holes formed primarily at lower redshifts), the observed change in slope and differential or “luminosity dependent density evolution” in the QLF is completely determined by the non-trivial, luminositydependent 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. Subject headings: quasars: general — galaxies: active — galaxies: evolution — cosmology: theory