Fueling Low‐Level AGN Activity through Stochastic Accretion of Cold Gas

Abstract

Using a simple description of feedback from black hole growth, we develop an analytic model for the fueling of Seyferts (low-luminosity active galactic nuclei [AGNs]) and their relation to their host galaxies, Eddington ratio distributions, and cosmological evolution. We derive a solution for the time evolution of accretion rates in a feedback-driven blast wave, applicable to large-scale outflows from bright quasars in galaxy mergers, low-luminosity AGNs, and black holes or neutron stars in supernova remnants. Under the assumption that cold gas stochastically accretes onto a central supermassive black hole at a rate set by the dynamics of that gas, our solution determines the evolution of Seyfert light curves. Using this model, we predict the Seyfert luminosity function, duty cycles and AGN "life-times,'' and the distribution of host morphologies, Eddington ratios, and obscuration as a function of AGN luminosity and black hole mass, and we find agreement with observations at z = 0. We consider the breakdown of the contribution from this mechanism and from stellar wind and virialized hot gas accretion and merger-driven activity. We also make specific predictions for the weak evolution of the Seyfert luminosity function; i.e., luminosity function of quiescent as opposed to merger-driven activity, as a function of redshift, and for changes in both the slope and scatter of the M-BH-sigma relation at low M-BH. Our modeling provides a quantitative and physical distinction between local, low-luminosity quiescent AGN activity and violent, merger-driven bright quasars. In our picture, the quiescent mode of fueling dominates over a wide range of luminosities (-15 greater than or similar to M-B greater than or similar to-22) at z = 0, where most black hole growth occurs in objects with M-BH less than or similar to 10(7) M-circle dot , in SO 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 "break'' in the luminosity function. Consequently, although non-merger-driven fueling is important for black hole growth and the M-BH-sigma relation at low M-BH, it does not significantly contribute to the black hole mass density of the universe or to cosmological backgrounds.