A radiative transfer model for the spiral galaxy M33
  We present the first radiative transfer (RT) model of a non-edge-on disk
galaxy in which the large-scale geometry of stars and dust is self-consistently
derived through fitting of multiwavelength imaging observations from the UV to
the submm. To this end we used the axi-symmetric RT model of Popescu et al. and
a new methodology for deriving geometrical parameters, and applied this to
decode the{spectral energy distribution (SED) of M33. We successfully account
for both the spatial and spectral energy distribution, with residuals typically
within $7\%$ in the profiles of surface brightness and within $8\%$ in the
spatially-integrated SED. We predict well the energy balance between absorption
and re-emission by dust, with no need to invoke modified grain properties, and
we find no submm emission that is in excess of our model predictions. We
calculate that $80\pm8\%$ of the dust heating is powered by the young stellar
populations. We identify several morphological components in M33, a nuclear, an
inner, a main and an outer disc, showing a monotonic trend in decreasing
star-formation surface-density ($\Sigma_{\rm SFR}$) from the nuclear to the
outer disc. In relation to surface density of stellar mass, the $\Sigma_{\rm
SFR}$ of these components define a steeper relation than the "main sequence" of
star-forming galaxies, which we call a "structurally resolved main sequence".
Either environmental or stellar feedback mechanisms could explain the slope of
the newly defined sequence. We find the star-formation rate to be ${\rm
SFR}=0.28^{+0.02}_{-0.01}{\rm M}_{\odot}{\rm yr}^{-1}$.
