Tiago Costa - April 7, 2017

The physics of AGN-driven outflows

Over their lifetime, active galactic nuclei (AGN) release an amount of energy
which exceeds the binding energy of their host galaxies by large factors ~100.
The extent to which the released energy couples to the interstellar- and
intergalactic media remains one of the greatest sources of uncertainty in
current models of galaxy formation, in great part because the underlying
physical mechanisms have not been identified. I will present a suite of
numerical simulations performed with the moving mesh code AREPO and the
radiation-hydrodynamic code RAMSES-RT, which have been tailored to explore a
wide range of AGN feedback models, including momentum-, energy- as well as
radiation pressure-driven winds in a full cosmological setting. I will argue
that under conditions in which supermassive black holes can grow at a rapid
rate, a momentum load > L/c is required in order to clear out the innermost
regions of the AGN host galaxy and drive large-scale outflows. This condition
places constraints on the nature of AGN-driven outflows, favouring the energy-
over the momentum-driven scenario. I will show that a momentum load > L/c may
also arise in configurations in which dusty gas in the galactic nucleus is
optically thick in the infrared (IR) and is pressurised by AGN radiation,
carrying important consequences to the origin of large-scale outflows and the
suppression of star formation in massive galaxies. Finally, I will show that the
balance of cooling and outflow timescales sets a characteristic velocity below
which the bulk of the outflow is cold and above which it remains hot, suggesting
that multi-phase structure in AGN-driven outflows may result from radiative
cooling of shocked outflowing gas.