Abstract
We review accretion mechanisms for powering the central engines of active galactic nuclei (AGN) and possible sources of fuel. Local sources, such as dense star clusters, require extreme set of parameters. Instead we argue that the interstellar matter in the main body of the host galaxy is channelled towards the centre and address the problem of angular momentum transport. Thin accretion disks are not a viable means of delivering fuel to luminous AGN on scales much larger than a parsec because of the long inflow time and effects of self-gravity. There are also serious obstacles to maintaining and regulating geometrically thick, hot accretion flows. We emphasize the role of non-axisymmetric perturbations of the gravitational potential on galactic scales and their triggers: galaxy interactions and internal self-gravitational instabilities. We outline a unified model for fuelling AGN, in which the inflow on large scales is driven by gravitational torques, and on small scales forms a mildly self-gravitating disk of clouds with inflow driven by magnetic torques or cloud–cloud collisions.
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