Clearing Out a Galaxy

Abstract

It is widely suspected that AGN activity ultimately sweeps galaxies clear of their gas. We work out the observable properties required to achieve this. Large-scale AGN-driven outflows should have kinetic luminosities $\sim \eta\le/2 \sim 0.05\le$ and momentum rates $\sim 20\le/c$, where $\le$ is the Eddington luminosity of the central black hole and $\eta\sim 0.1$ its radiative accretion efficiency. This creates an expanding two-phase medium in which molecular species coexist with hot gas, which can persist after the central AGN has switched off. This picture predicts outflow velocities $\sim 1000 - 1500$ km\,s$^{-1}$ and mass outflow rates up to $4000 \msun\,{\rm yr}^{-1}$ on kpc scales, fixed mainly by the host galaxy velocity dispersion (or equivalently black hole mass). All these features agree with those of outflows observed in galaxies such as Mrk231. This strongly suggests that AGN activity is what sweeps galaxies clear of their gas on a dynamical timescale and makes them red and dead. We suggest future observational tests of this picture.