The Heating of Mid-Infrared Dust in the Nearby Galaxy M33: A Testbed for
  Tracing Galaxy Evolution
  Because the 8 {\mu}m polycyclic aromatic hydrocarbon (PAH) emission has been
found to correlate with other well-known star formation tracers, it has widely
been used as a star formation rate (SFR) tracer. There are, however, studies
that challenge the accuracy and reliability of the 8 {\mu}m emission as a SFR
tracer. Our study, part of the Herschel M33 Extended Survey (HERM33ES) open
time key program, aims at addressing this issue by analyzing the infrared
emission from the nearby spiral galaxy M33 at the high spatial scale of 75 pc.
Combining data from the Herschel Space Observatory and the Spitzer Space
Telescope we find that the 8 {\mu}m emission is better correlated with the 250
{\mu}m emission, which traces cold interstellar gas, than with the 24 {\mu}m
emission. The L(8)/L(24) ratio is highly depressed in 24 {\mu}m luminous
regions, which correlate with known HII regions. We also compare our results
with the dust emission models by Draine & Li (2007). We confirm that the
depression of 8 {\mu}m PAH emission near star-forming regions is higher than
what is predicted by models; this is possibly an effect of increased stellar
radiation from young stars destroying the dust grains responsible for the 8
{\mu}m emission as already suggested by other authors. We find that the
majority of the 8 {\mu}m emission is fully consistent with heating by the
diffuse interstellar medium, similar to what recently determined for the dust
emission in M31 by Draine at al. (2013). We also find that the fraction of 8
{\mu}m emission associated with the diffuse interstellar radiation field ranges
between 60% and 80% and is 40% larger than the diffuse fraction at 24 {\mu}m.
