Driving gas shells with radiation pressure on dust in
  radiation-hydrodynamic simulations
  We present radiation-hydrodynamic simulations of radiatively-driven gas
shells launched by bright active galactic nuclei (AGN) in isolated dark matter
haloes. Our goals are (1) to investigate the ability of AGN radiation pressure
on dust to launch galactic outflows and (2) to constrain the efficiency of
infrared (IR) multi-scattering in boosting outflow acceleration. Our
simulations are performed with the radiation-hydrodynamic code RAMSES-RT and
include both single- and multi-scattered radiation pressure from an AGN,
radiative cooling and self-gravity. Since outflowing shells always eventually
become transparent to the incident radiation field, outflows that sweep up all
intervening gas are likely to remain gravitationally bound to their halo even
at high AGN luminosities. The expansion of outflowing shells is well described
by simple analytic models as long as the shells are mildly optically thick to
IR radiation. In this case, an enhancement in the acceleration of shells
through IR multi-scattering occurs as predicted, i.e. a force dP/dt = tau_IR
L/c is exerted on the gas. For high optical depths tau_IR > 50, however,
momentum transfer between outflowing optically thick gas and IR radiation is
rapidly suppressed, even if the radiation is efficiently confined. At high
tau_IR, the characteristic flow time becomes shorter than the required trapping
time of IR radiation such that the momentum flux dP/dt << tau_IR L/c. We argue
that while unlikely to unbind massive galactic gaseous haloes, AGN radiation
pressure on dust could play an important role in regulating star formation and
black hole accretion in the nuclei of massive compact galaxies at high
redshift.
