Radiation pressure in galactic disks: stability, turbulence, and winds
  in the single-scattering limit
  The radiation force on dust grains may be dynamically important in driving
turbulence and outflows in rapidly star-forming galaxies. Recent studies focus
on the highly optically-thick limit relevant to the densest ultra-luminous
galaxies and super star clusters, where reprocessed infrared photons provide
the dominant source of electromagnetic momentum. However, even among starburst
galaxies, the great majority instead lie in the so-called "single-scattering"
limit, where the system is optically-thick to the incident starlight, but
optically-thin to the re-radiated infrared. In this paper we present a
stability analysis and multidimensional radiation-hydrodynamic simulations
exploring the stability and dynamics of isothermal dusty gas columns in this
regime. We describe our algorithm for full angle-dependent radiation transport
based on the discontinuous Galerkin finite element method. For a range of
near-Eddington fluxes, we show that the medium is unstable, producing
convective-like motions in a turbulent atmosphere with a scale height
significantly inflated compared to the gas pressure scale height and
mass-weighted turbulent energy densities of $\sim 0.01-0.1$ of the midplane
radiation energy density, corresponding to mass-weighted velocity dispersions
of Mach number $\sim 0.5-2$. Extrapolation of our results to optical depths of
$10^3$ implies maximum turbulent Mach numbers of $\sim20$. Comparing our
results to galaxy-averaged observations, and subject to the approximations of
our calculations, we find that radiation pressure does not contribute
significantly to the effective supersonic pressure support in star-forming
disks, which in general are substantially sub-Eddington. We further examine the
time-averaged vertical density profiles in dynamical equilibrium and comment on
implications for radiation-pressure-driven galactic winds.
