Does the Thermal Disk Instability Operate in Active Galactic Nuclei?

We examine all possible stationary, optically thick, geometrically thin accretion disk models relevant for active galactic nuclei (AGN) and identify the physical regimes in which they are stable against the thermal-viscous hydrogen ionization instability. Self-gravity and irradiation effects are included. We find that most if not all AGN disks are unstable. Observed AGN therefore represent the outburst state, although some or all quasars could constitute a steady population having markedly higher fueling rates than other AGN. This finding has important implications for the AGN mass supply and for the presence of supermassive black holes in nearby spirals.