When a relativistic AGN jet enters the Interstellar Medium of its host galaxy, an internal backflow from the hotspot develops. We demonstrate from first principles (Crocco theorem) that this is a necessary consequence of mechanical jet emission. The flow lines bring very low angular monetum gas towards the central accreting region, and could ultimately feed the central Supermassive Black Hole (SMBH). We have studied in detail the circulation of gas inside the cooon performing an exhustive set of numerical simulations, and developed a hydrodynamic model of this flow. We show that this circulation arises because of the presence of entropy and density gradients in two critical points: near the hot spot and the meridional plane of the cocoon. We find simple solutions for the streamlines within the bow shock, solving the appropriate fluid dynamic equations, and we use these solutions and the simulations to predict the mass inflow rates towards the central regions. We find that this how backflow affects the accretion disc by compressing it and thus modifyng the efficency of magnetic field lines diffusion, whch is directly connected to the jet's mechanical power. The time scales of these backflows are only weakly dependent on the jet/ISM parameters. The modulation of jets power results into a self-regulatory mechanism of AGN activity, that is not directly controlled by, but possibly controls,  the star formation rate within the central circumnuclear disk, and ultimately its feedback on star formation quenching.

http://www.oats.inaf.it/index.php/it/seminars.html