  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.
