Efficient 3D NLTE dust radiative transfer with SKIRT
  We present an updated version of SKIRT, a 3D Monte Carlo radiative transfer
code developed to simulate dusty galaxies. The main novel characteristics of
the SKIRT code are the use of a stellar foam to generate random positions, an
efficient combination of eternal forced scattering and continuous absorption,
and a new library approach that links the radiative transfer code to the DustEM
dust emission library. This approach enables a fast, accurate and
self-consistent calculation of the dust emission of arbitrary mixtures of
transiently heated dust grains and polycyclic aromatic hydrocarbons, even for
full 3D models containing millions of dust cells. We have demonstrated the
accuracy of the SKIRT code through a set of simulations based on the edge-on
spiral galaxy UGC 4754. The models we ran were gradually refined from a smooth,
2D, LTE model to a fully 3D model that includes NLTE dust emission and a clumpy
structure of the dusty ISM. We find that clumpy models absorb UV and optical
radiation less efficiently than smooth models with the same amount of dust, and
that the dust in clumpy models is on average both cooler and less luminous. Our
simulations demonstrate that, given the appropriate use of optimization
techniques, it is possible to efficiently and accurately run Monte Carlo
radiative transfer simulations of arbitrary 3D structures of several million
dust cells, including a full calculation of the NLTE emission by arbitrary dust
mixtures.
