Variation in the dust emissivity index across M33 with Herschel and
  Spitzer (HerM33es)
  We study the wavelength dependence of the dust emission as a function of
position and environment across the disk of M33 at a linear resolution of 160
pc using Spitzer and Herschel photometric data. Expressing the emissivity of
the dust as a power law, the power-law exponent (beta) is estimated from two
independent approaches designed to properly treat the degeneracy between beta
and the dust temperature. Both beta and the dust temperature are higher in the
inner disk than in the outer disk, contrary to reported beta-T
anti-correlations found in other sources. In the cold + warm dust model, the
warm component and the ionized gas (Halpha) have a very similar distribution
across the galaxy, demonstrating that the model separates the components in an
appropriate fashion. The flocculent spiral arms and the dust lanes are evident
in the map of the cold component. Both cold and warm dust column densities are
high in star forming regions and reach their maxima toward the giant star
forming complexes NGC604 and NGC595. beta declines from close to 2 in the
center to about 1.3 in the outer disk. beta is positively correlated with star
formation and with molecular gas column, as traced by Halpha and CO emission.
The lower dust emissivity index in the outer parts of M33 is likely related to
the reduced metallicity (different grain composition) and possibly different
size distribution. It is not due to the decrease in stellar radiation field or
temperature in a simple way because the FIR-bright regions in the outer disk
also have a low beta. Like most spirals, M33 has a (decreasing) radial gradient
in star formation and molecular-to-atomic gas ratio such that the regions
bright in Halpha or CO tend to trace the inner disk, making it difficult to
distinguish between their effects on the dust.
