Fueling Low-Level AGN Activity Through the Stochastic Accretion of Cold Gas

Abstract

Using a simple description of feedback from black hole (BH) growth, we develop an analytic model for the fueling of Seyferts (low-luminosity AGN) and their relation to their host galaxies, Eddington ratio distributions, and cosmological evolution. We derive a solution for evolution of accretion rates in a feedback-driven blast wave, applicable to large-scale outflows from bright quasars in galaxy mergers, low-luminosity AGN, and BHs in supernova remnants. Under the assumption that cold gas stochastically accretes onto a central SMBH at a rate set by the dynamics of that gas, our solution determines the Seyfert light curve evolution. We use this to predict the Seyfert luminosity function (LF), duty cycles, lifetimes, and distribution of host morphologies, Eddington ratios, and obscuration as a function of AGN luminosity and mass, and find agreement with observations at z=0. We consider the breakdown of contributions from this and stellar wind accretion and merger-driven activity. We make specific predictions for the weak evolution of the Seyfert LF with redshift (LF of non-merger driven activity), and for changes in both the slope and scatter of the M_BH-sigma relation at low-M_BH. Quiescent fueling dominates over a wide luminosity range (-14 > M_B > -22) at z=0, where most BH growth occurs around M_BH~10^7 M_sun, in S0 and Sa/b galaxies. However, quasar activity from gas-rich mergers evolves more rapidly with redshift, and by z=1 quiescent fueling is important only at luminosities an order of magnitude or more below the LF break. Although non-merger driven fueling is important for BH growth and the M_BH-sigma relation at low M_BH, it does not significantly contribute to the total BH mass density or cosmological backgrounds.