Accretion Disks and the Link Between an AGN and its Host Galaxy

Abstract

We examine the possible role of accretion disks in bringing matter from the interstellar medium into the central regions of an active galactic nucleus (AGN). Thin axisymmetric accretion disks are not a viable means of transporting fuel to luminous AGNs on scales larger than a fraction of a parsec, because 1) the inflow time scale is too long; and 2) disks carrying the required fuel supply would become self-gravitating, leading to fragmentation and probably star formation. We consider the effects of star formation, concluding that energy input from stellar winds and supernovae is probably inadequate to increase the disk thickness to the point where disk accretion becomes viable. There are also serious obstacles to maintaining and regulating the energetics of geometrically thick, hot accretion flows, although the possibility of fuelling some AGNs through cooling flows cannot be ruled out. In this paper and the companion paper by Frank, Shlosman and Begelman (these proceedings), we outline a unified model for fuelling AGNs, in which the inflow is driven by global nonaxisymmetric gravitational instabilities on large scales, and on small scales forms a mildly self-gravitating disk of clouds in which angular momentum is transported by cloud-cloud collisions.