Do We Expect Most AGN to Live in Disks?

Abstract

Recent observations have indicated that a large fraction of the low to intermediate luminosity AGN population lives in disk-dominated hosts, while the more luminous quasars live in bulge-dominated hosts, in conflict with some previous model predictions. We therefore build and compare a semi-empirical model for AGN fueling which accounts for both merger and non-merger 'triggering.' In particular, we show that the 'stochastic accretion' model - in which fueling in disk galaxies is essentially a random process arising whenever dense gas clouds reach the nucleus - provides a good match to the present observations at low/intermediate luminosities. However it falls short of the high-luminosity population. We combine this with models for major merger-induced AGN fueling, which lead to rarer but more luminous events, and predict the resulting abundance of disk-dominated and bulge-dominated AGN host galaxies as a function of luminosity and redshift. We compile and compare observational constraints from z~0-2. The models and observations generically show a transition from disk to bulge dominance in hosts near the Seyfert-quasar transition, at all redshifts. 'Stochastic' fueling dominates AGN by number (dominant at low luminosity), and dominates BH growth below the knee in the present-day BH mass function (<10^7 M_sun). However it accounts for just ~10% of BH mass growth at masses >10^8 M_sun. In total, fueling in disky hosts accounts for ~30% of the total AGN luminosity density/BH mass density. The combined model also accurately predicts the AGN luminosity function and clustering/bias as a function of luminosity and redshift; however, we argue that these are not sensitive probes of BH fueling mechanisms.