2. AGN FUELING THROUGH DENSE NUCLEAR STAR CLUSTERS

To estimate more quantitatively the fueling problem, let us consider the typical luminosity and power of an active nucleus, that can be of the order or higher than 10⁴⁶ erg/s. If we assume a mass-to-energy conversion efficiency ~ 10% (L = dM/dt c² ), then the mass accretion rate dM/dt should be:

dM/dt ~ 1.7 (0.1/) (L/10⁴⁶ erg/s) M/yr

If the duty cycle of the AGN is of the order of 10⁸ yr, then a total mass up to 2 10⁸ M should be available. It is a significant fraction of the gas content of a typical galaxy, like the Milky Way! The time-scale to drive such a large mass to the center is likely to be larger than 1 Gyr.

For the mass to infall to the center, it must lose its angular momentum. Could this be due to viscous torques? In a geometrically thin accretion disk, one can consider the gas subsonic viscosity, where the viscous stress is modelled proportional () to the internal pressure, with a factor < 1. This can only gather in 1 Gyr the gas within 4 pc typically (e.g. Shlosman et al 1989, Phinney 1994). This shows that viscous torques will not couple the large-scale galaxy to the nucleus, only the very nuclear regions could play a role through viscous torques.