High-resolution, 3D radiative transfer modelling. II. The early-type
  spiral galaxy M81
  Interstellar dust absorbs stellar light very efficiently and thus shapes the
energetic output of galaxies. Studying the impact of different stellar
populations on the dust heating remains hard because it requires decoupling the
relative geometry of stars and dust, and involves complex processes as
scattering and non-local dust heating. We aim to constrain the relative
distribution of dust and stellar populations in the spiral galaxy M81 and
create a realistic model of the radiation field that describes the
observations. Investigating the dust-starlight interaction on local scales, we
want to quantify the contribution of young and old stellar populations to the
dust heating. We aim to standardise the setup and model selection of such
inverse radiative transfer simulations so this can be used for comparable
modelling of other nearby galaxies. We present a semi-automated radiative
transfer modelling pipeline that implements the necessary steps such as the
geometric model construction and the normalisation of the components through an
optimisation routine. We use the Monte Carlo radiative transfer code SKIRT to
calculate a self-consistent, panchromatic model of the interstellar radiation
field. By looking at different stellar populations independently, we can
quantify to what extent different stellar age populations contribute to the
dust heating. Our method takes into account the effects of non-local heating.
We obtain a realistic 3D radiative transfer model of the face-on galaxy M81. We
find that only 50.2\% of the dust heating can be attributed to young stellar
populations. We confirm a tight correlation between the specific star formation
rate and the heating fraction by young stellar populations, both in sky
projection and in 3D, also found for radiative transfer models of M31 and M51.
We conclude that... (abridged)
