Radiation Feedback in ULIRGS: Are Photons Movers and Shakers?
  We use our variable Eddington tensor (VET) radiation hydrodynamics code to
perform two-dimensional simulations to study the impact of radiation forces on
atmospheres composed of dust and gas. Our setup closely follows that of
Krumholz & Thompson, assuming that dust and gas are well-coupled and that the
radiation field is characterized by blackbodies with temperatures >~ 80 K, as
might be found in ultraluminous infrared galaxies. In agreement with previous
work, we find that Rayleigh-Taylor instabilities develop in radiation supported
atmospheres, leading to inhomogeneities that limit momentum exchange between
radiation and dusty gas, and eventually providing a near balance of the
radiation and gravitational forces. However, the evolution of the velocity and
spatial distributions of the gas differs significantly from previous work,
which utilized a less accurate flux-limited diffusion (FLD) method. Our VET
simulations show continuous net acceleration of the gas, with no steady-state
reached by the end of the simulation. In contrast, FLD results show little net
acceleration of the gas and settle in to a quasi-steady, turbulent state with
low velocity dispersion. The discrepancies result primarily from the inability
of FLD to properly model the variation of the radiation field around structures
that are less than a few optical depths across. We conclude that radiation
feedback remains a viable mechanism for driving high-Mach number turbulence. We
discuss implications for observed systems and global numerical simulations of
feedback, but more realistic setups are needed to make robust observational
predictions and assess the prospect of launching outflows with radiation.
