Self-gravitating accretion disks in active galactic nuclei

Abstract

Accretion onto supermassive black holes (SBHs) is widely believed to be responsible for the phenomena of active galactic nuclei (AGN). While there has been considerable theoretical work on the inner regions of such accretion flows, little is known about the outer boundaries of AGN. Except for the broad and narrow emission lines, the observational constraints are weak, and evidence linking the emission-line regions to the accretion flow is ambiguous at best. It is not even known whether the accretion flow is fuelled by mass loss from a dense cluster of stars surrounding the SBH, or enters the nucleus from the galactic interstellar medium (ISM). Here we adopt the latter view, assuming that most of the incoming fuel forms a thin accretion disk at distances of ∼ 10–10³ pc from the SBH. Such a disk must be vertically self-gravitating. We analyse some of its thermal and dynamical properties, and conclude that its energetics is likely to be dominated by backscattered AGN radiation. We also discuss the conditions under which Jeans fragmentation of such a disk into weakly interacting cloudlets can be avoided, and give a necessary condition for disk fragmentation to occur.