Radiative transfer of ionizing radiation through gas and dust: grain
  charging in star forming regions
  The presence of charged dust grains is known to have a profound impact on the
physical evolution of the multiphase interstellar medium (ISM). Despite its
importance, this process is still poorly explored in numerical simulations due
to its complex physics and the tight dependence on the environment. Here we
introduce a novel implementation of grain charging in the cosmological
radiative transfer code CRASH. We first benchmark the code predictions on a
series of idealized dusty HII regions created by a single star, in order to
assess the impact of grain properties on the resulting spatial distribution of
charges. Second, we perform a realistic radiative transfer simulation of a star
forming region extracted from a dusty galaxy evolving in the Epoch of
Reionization. We find that $\sim 13$ % of the total dust mass gets negatively
charged, mainly silicate and graphite grains of radius micron. A complex
spatial distribution of grain charges is also found, primarily depending on the
exposure to stellar radiation and strongly varying along different lines of
sight, as a result of radiative transfer effects. We finally assess the impact
of grain properties (both chemical composition and size) on the resulting
charge distribution. The new implementation described here will open up a wide
range of possible studies investigating the physical evolution of the dusty
ISM, nowadays accessible to observations of high and low redshift galaxies.
