Warm absorbers: supermassive black hole feeding, and Compton-thick AGN
  Warm absorbers are found in many AGN and consist of clouds moving at moderate
radial velocities, showing complex ionization structures and having moderate to
large column densities. Using 1D numerical calculations, we confirm earlier
suggestions that the energy released by an AGN pushes the surrounding gas
outward in a bubble until this reaches transparency. Typical AGN episode
durations of $5\times 10^4$ yr supply enough energy for this, except in very
gas-rich and/or very compact galaxies, such as those in the early Universe. In
those galaxies, the AGN might remain hidden for many periods of activity,
hiding the black hole growth. The typical radii of $0.1-1$ kpc, velocities of
$100-1000$ km s$^{-1}$ and resulting optical depths are consistent with
observations of warm absorbers. The resulting structure is a natural outcome of
outflows driven by AGN buried in an optically thick gas envelope, and has a
total mass comparable to the final $M -\sigma$ mass the central supermassive
black hole will eventually reach.These results suggest that AGN can feed very
efficiently by agitating this surrounding dense material. This may not be easy
to observe, as this gas is Compton thick along many sightlines. The infall may
produce episodic star formation in the centre, building up nuclear star
clusters simultaneously with the growth of the central black hole.
