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Recent observations have indicated that a large fraction of the low tointermediate luminosity AGN population lives in disk-dominated hosts, while themore luminous quasars live in bulge-dominated hosts, in conflict with someprevious model predictions. We therefore build and compare a semi-empiricalmodel for AGN fueling which accounts for both merger and non-merger'triggering.' In particular, we show that the 'stochastic accretion' model - inwhich fueling in disk galaxies is essentially a random process arising wheneverdense gas clouds reach the nucleus - provides a good match to the presentobservations at low/intermediate luminosities. However it falls short of thehigh-luminosity population. We combine this with models for majormerger-induced AGN fueling, which lead to rarer but more luminous events, andpredict the resulting abundance of disk-dominated and bulge-dominated AGN hostgalaxies as a function of luminosity and redshift. We compile and compareobservational constraints from z~0-2. The models and observations genericallyshow a transition from disk to bulge dominance in hosts near the Seyfert-quasartransition, at all redshifts. 'Stochastic' fueling dominates AGN by number(dominant at low luminosity), and dominates BH growth below the knee in thepresent-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 hostsaccounts for ~30% of the total AGN luminosity density/BH mass density. Thecombined model also accurately predicts the AGN luminosity function andclustering/bias as a function of luminosity and redshift; however, we arguethat these are not sensitive probes of BH fueling mechanisms.

Do We Expect Most AGN to Live in Disks?