Spatially resolving the dust properties and submillimetre excess in M 33
  The relative abundance of the dust grain types in the interstellar medium
(ISM) is directly linked to physical quantities that trace the evolution of
galaxies. We study the dust properties of the whole disc of M33 at spatial
scales of ~170 pc. This analysis allows us to infer how the relative dust grain
abundance changes with the conditions of the ISM, study the existence of a
submillimetre excess and look for trends of the gas-to-dust mass ratio (GDR)
with other physical properties of the galaxy. For each pixel in the disc of M33
we fit the infrared SED using a physically motivated dust model that assumes an
emissivity index beta close to 2. We derive the relative amount of the
different dust grains in the model, the total dust mass, and the strength of
the interstellar radiation field (ISRF) heating the dust at each spatial
location. The relative abundance of very small grains tends to increase, and
for big grains to decrease, at high values of Halpha luminosity. This shows
that the dust grains are modified inside the star-forming regions, in agreement
with a theoretical framework of dust evolution under different physical
conditions. The radial dependence of the GDR is consistent with the shallow
metallicity gradient observed in this galaxy. The strength of the ISRF derived
in our model correlates with the star formation rate in the galaxy in a pixel
by pixel basis. Although this is expected it is the first time that a
correlation between both quantities is reported. We produce a map of
submillimetre excess in the 500 microns SPIRE band for the disc of M33. The
excess can be as high as 50% and increases at large galactocentric distances.
We further study the relation of the excess with other physical properties of
the galaxy and find that the excess is prominent in zones of diffuse ISM
outside the main star-forming regions, where the molecular gas and dust surface
density are low.
