Introduction

A two stage mechanism for fuelling AGN is proposed which makes use of stellar dynamical and gas dynamical instabilities (Shlosman et al. 1989). First, a stellar bar sweeps the interstellar material inwards as a consequence of the gas losing angular momentum to the bar. In a second stage, the gaseous disc accumulated in the nuclear region of the host galaxy, goes bar unstable again and the material flows further in. The main criterion for the occurence of the second instability is that the mass of the gaseous disc must exceed some critical fraction of the total mass of the host galaxy. This critical mass fraction is of the order of a tenth or so according to our estimates. The inflowing gas may eventually join a viscosity-driven accretion disc if a black hole was already present or lead to its formation. If the host galaxy is relatively gas rich, but the disc formed during the first stage does not exceed the critical mass, or if the inflow of gas is halted around resonances, a nuclear starburst may follow. This mechanism may explain the association of bars and rings with nuclear activity and the dichotomy between AGN and starburst nuclei.

Gas in galaxies

The atomic and molecular gas content of spiral galaxies is observed to be in the range 108 – 1010 M⊙ and to peak at Hubble types Sb-Sbc (Haynes & Giovanelli 1984, Verter 1987).