### 2. AGN FUELING THROUGH DENSE NUCLEAR STAR CLUSTERS

**2.1. Fueling: why is there a problem?**
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 10^{46} erg/s.
If we assume a mass-to-energy conversion efficiency
~ 10% (L = dM/dt
c^{2} ), then the
mass accretion rate dM/dt should be:

dM/dt ~ 1.7 (0.1/)
(L/10^{46} erg/s)
M_{}/yr

If the duty cycle of the AGN is of the order of
10^{8} yr, then a total mass up to 2 10^{8}
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.