The source of fueling depends on the strength and luminosity of the AGN:
- for low luminosity AGN and Seyferts, only stars from a dense nuclear cluster are sufficient (through tidal distorsions and stellar collisions),
- for high luminosity AGN and quasars, large accretion rates are required, which involve large-scale gravitational instabilities. Those drive gas towards the center that trigger big starbursts, and the coeval compact cluster just formed can provide the fuel through mass loss of young stars and supernovae. This gas must have been driven from the galactic disks, through internal gravitational instabilities (bars, spirals), more generally the consequence of interactions and mergers.
Galaxy disks are in general far from stationary equilibrium, most often subject to m = 2 and m = 1 instabilities. These non-axisymmetric instabilities produce gravity torques, that drive the gas inwards. When the mass concentration towards the center is large enough, a secondary bar can decouple and rotate with a higher pattern speed. Embedded bars or non-axisymmetric structures can take over the gas flow to fuel the nucleus. The fueling is therefore favored in early-type objects.
Interactions and mergers also produce bars and non-axisymmetric structures, that fuel the nucleus through their gravity torques. They first trigger huge starbursts in the merger center, that could afterwards fuel the AGN. Since internal instabilities, external trigger and hierachical merging both produce the bulges and fuel the nucleus, it is natural to expect a proportionality ratio in their masses.
Acknowledgments
I am very grateful to the organisers for inviting me to give these lectures, in a very friendly and scientifically exciting ambiance.