Dusty Cloud Acceleration by Radiation Pressure in Rapidly Star-Forming
  Galaxies
  We perform two-dimensional and three-dimensional radiation hydrodynamic
simulations to study cold clouds accelerated by radiation pressure on dust in
the environment of rapidly star-forming galaxies dominated by infrared flux. We
utilize the reduced speed of light approximation to solve the
frequency-averaged, time-dependent radiative transfer equation. We find that
radiation pressure is capable of accelerating the clouds to hundreds of
kilometers per second while remaining dense and cold, consistent with
observations. We compare these results to simulations where acceleration is
provided by entrainment in a hot wind, where the momentum injection of the hot
flow is comparable to the momentum in the radiation field. We find that the
survival time of the cloud accelerated by the radiation field is significantly
longer than that of a cloud entrained in a hot outflow. We show that the
dynamics of the irradiated cloud depends on the initial optical depth,
temperature of the cloud, and the intensity of the flux. Additionally, gas
pressure from the background may limit cloud acceleration if the density ratio
between the cloud and background is $\lesssim 10^{2}$. In general, a 10
pc-scale optically thin cloud forms a pancake structure elongated perpendicular
to the direction of motion, while optically thick clouds form a filamentary
structure elongated parallel to the direction of motion. The details of
accelerated cloud morphology and geometry can also be affected by other
factors, such as the cloud lengthscale, the reduced speed of light
approximation, spatial resolution, initial cloud structure, and the
dimensionality of the run, but these have relatively little affect on the cloud
velocity or survival time.
