Evidence for an extranuclear AGN fuel source

The present paradigm for AGNs consists of a supermassive black hole (the ``central engine'') plus material whose gravitational energy is extracted as it falls into the central engine (the ``fuel''). In this paper, I review the evidence regarding the popular notion that the fuel source is located outside the nucleus (at radii where the gravitational force of the central engine is small compared to that of the host galaxy). The only direct kinematic evidence for infalling fuel is the recent observation of systematically redshifted HI λ21 cm absorption-lines seen against nuclear radio sources in radio-loud E and SO galaxies. The kinematics of the ionized gas in the ``Narrow-Line-Region'' (NLR) in AGNs provides indirect kinematic evidence for an extranuclear fuel source. In Seyfert galaxies the NLR kinematics are consistent with (but do not demand) freefall into the nucleus. In powerful radio galaxies, the alignment between the NLR rotation axis and the radio ejection axis strongly suggests that gas (and its associated angular momentum) is transported inward over many decades in radius. I also review nonkinematic evidence for an extranuclear fuel source, including the possible connection between cooling flows and radio emission in giant ellipticals, the presence of large central molecular gas concentrations in some Seyfert galaxies, and the preference of Seyfert nuclei for ovally-distorted host galaxies. I briefly summarize the evidence for a link between the AGN phenomenon and galaxy interactions. This relationship provides indirect but highly suggestive evidence that the extranuclear environment plays a major role in fueling AGNs.