The energetics of AGN radiation pressure-driven outflows
  The increasing observational evidence of galactic outflows is considered as a
sign of active galactic nucleus (AGN) feedback in action. However, the physical
mechanism responsible for driving the observed outflows remains unclear, and
whether it is due to momentum, energy, or radiation is still a matter of
debate. The observed outflow energetics, in particular the large measured
values of the momentum ratio ($\dot{p}/(L/c) \sim 10$) and energy ratio
($\dot{E}_k/L \sim 0.05$), seems to favour the energy-driving mechanism; and
most observational works have focused their comparison with wind energy-driven
models. Here we show that AGN radiation pressure on dust can adequately
reproduce the observed outflow energetics (mass outflow rate, momentum flux,
and kinetic power), as well as the scalings with luminosity, provided that the
effects of radiation trapping are properly taken into account. In particular,
we predict a sub-linear scaling for the mass outflow rate ($\dot{M} \propto
L^{1/2}$) and a super-linear scaling for the kinetic power ($\dot{E}_k \propto
L^{3/2}$), in agreement with the observational scaling relations reported in
the most recent compilation of AGN outflow data. We conclude that AGN radiative
feedback can account for the global outflow energetics, at least equally well
as the wind energy-driving mechanism, and therefore both physical models should
be considered in the interpretation of future AGN outflow observations.
