AGN activity: Self-regulation from backflow
  We study the internal circulation within the cocoon carved out by the
relativistic jet emanating from an AGN within the ISM of its host galaxy.
Firstly, we develop a model for the origin of the internal flow, noticing that
a significant increase of large scale velocity circulation within the cocoon
arises as significant gradients in the density and entropy are created near the
hot spot (a consequence of Crocco's vorticity generation theorem). We find
simple and accurate approximate solutions for the large scale flow,showing that
a backflow towards the few inner parsec region develops. We solve the
appropriate fluid dynamic equations, and we use these solutions to predict the
mass inflow rates towards the central regions. We then perform a series of 2D
simulations of the propagation of jets using FLASH 2.5, in order to validate
the predictions of our model. The backflows which arise supply the central AGN
region with very low angular momentum gas, at average rates of the order of
$0.1-0.8 \rm{M}_{\odot} \rm{yr.}^{-1}$, the exact value seen to be strongly
dependent on the central ISM density (for fixed input jet power). The time
scales of these inflows are apparently weakly dependent on the jet/ISM
parameters, and are of the order of $3-5\times 10^{7} \rm{yrs}$. We then argue
that these backflows could (at least partially) feed the AGN, and provide a
self-regulatory mechanism of AGN activity, that is not directly controlled by,
but instead controls, the star formation rate within the central circumnuclear
disk.
