Arepo-MCRT: Monte Carlo Radiation Hydrodynamics on a Moving Mesh
  We present Arepo-MCRT, a novel Monte Carlo radiative transfer (MCRT)
radiation-hydrodynamics (RHD) solver for the unstructured moving-mesh code
Arepo. Our method is designed for general multiple scattering problems in both
optically thin and thick conditions. We incorporate numerous efficiency
improvements and noise reduction schemes to help overcome efficiency barriers
that typically inhibit convergence. These include continuous absorption and
energy deposition, photon weighting and luminosity boosting, local packet
merging and splitting, path-based statistical estimators, conservative
(face-centered) momentum coupling, adaptive convergence between time steps,
implicit Monte Carlo algorithms for thermal emission, and discrete-diffusion
Monte Carlo techniques for unresolved scattering, including a novel advection
scheme. We primarily focus on the unique aspects of our implementation and
discussions of the advantages and drawbacks of our methods in various
astrophysical contexts. Finally, we consider several test applications
including the levitation of an optically thick layer of gas by trapped infrared
radiation. We find that the initial acceleration phase and revitalized second
wind are connected via self-regulation of the RHD coupling, such that the RHD
method accuracy and simulation resolution each leave important imprints on the
long-term behavior of the gas.
