Driving Outflows with Relativistic Jets and the Dependence of AGN
  Feedback Efficiency on ISM Inhomogeneity
  We examine the detailed physics of the feedback mechanism by relativistic AGN
jets interacting with a two-phase fractal interstellar medium in the kpc-scale
core of galaxies using 29 3D grid-based hydrodynamical simulations. The
feedback efficiency, as measured by the amount of cloud-dispersal generated by
the jet-ISM interactions, is sensitive to the maximum size of clouds in the
fractal cloud distribution but not to their volume filling factor. Feedback
ceases to be efficient for Eddington ratios P_jet/L_edd<10^-4, although systems
with large cloud complexes ~50 pc require jets of Eddington ratio in excess of
10^-2 to disperse the clouds appreciably. Based on measurements of the bubble
expansion rates in our simulations we argue that sub-grid AGN prescriptions
resulting in negative feedback in cosmological simulations without a
multi-phase treatment of the ISM are good approximations if the volume filling
factor of warm phase material is less than 0.1 and the cloud complexes are
smaller than ~25 pc. We find that the acceleration of the dense embedded clouds
is provided by the ram pressure of the high velocity flow through the porous
channels of the warm phase, flow that has fully entrained the shocked hot-phase
gas it has swept up, and is additionally mass-loaded by ablated cloud material.
This mechanism transfers 10% to 40% of the jet energy to the cold and warm gas,
accelerating it within 10 to 100 Myr to velocities that match those observed in
a range of high and low redshift radio galaxies hosting powerful radio jets.
