RAMSES-RT: Radiation hydrodynamics in the cosmological context
  We present a new implementation of radiation hydrodynamics (RHD) in the
adaptive mesh refinement (AMR) code RAMSES. The multi-group radiative transfer
(RT) is performed on the AMR grid with a first-order Godunov method using the
M1 closure for the Eddington tensor, and is coupled to the hydrodynamics via
non-equilibrium thermochemistry of hydrogen and helium. This moment-based
approach has the large advantage that the computational cost is independent of
the number of radiative sources - it can even deal with continuous regions of
emission such as bound-free emission from gas. As it is built directly into
RAMSES, the RT takes natural advantage of the refinement and parallelization
strategies already in place. Since we use an explicit advection solver for the
radiative transport, the time step is restricted by the speed of light - a
severe limitation that can be alleviated using the so--called "reduced speed of
light" approximation. We propose a rigorous framework to assess the validity of
this approximation in various conditions encountered in cosmology and galaxy
formation. We finally perform with our newly developed code a complete suite of
RHD tests, comparing our results to other RHD codes. The tests demonstrate that
our code performs very well and is ideally suited for exploring the effect of
radiation on current scenarios of structure and galaxy formation.
