Quantifying the Heating Sources for Mid-infrared Dust Emissions in
  Galaxies: The Case of M 81
  With the newly available SPIRE images at 250 and 500 micron from Herschel
Space Observatory, we study quantitative correlations over a sub-kpc scale
among three distinct emission components in the interstellar medium of the
nearby spiral galaxy M 81 (NGC 3031): (a) $I_{8}$ or $I_{24}$, the surface
brightness of the mid-infrared emission observed in the Spitzer IRAC 8 or MIPS
24 micron band, with $I_8$ and $I_{24}$ being dominated by the emissions from
Polycyclic Aromatic Hydrocarbons (PAHs) and very small grains (VSGs) of dust,
respectively; (b) $I_{500}$, that of the cold dust continuum emission in the
Herschel SPIRE 500 micron band, dominated by the emission from large dust
grains heated by evolved stars, and (c) $I_{{\rm H}\alpha}$, a nominal surface
brightness of the H$\alpha$ line emission, from gas ionized by newly formed
massive stars. The results from our correlation study, free from any assumption
on or modeling of dust emissivity law or dust temperatures, present solid
evidence for significant heating of PAHs and VSGs by evolved stars. In the case
of M 81, about 67% (48%) of the 8 micron (24 micron) emission derives its
heating from evolved stars, with the remainder attributed to radiation heating
associated with ionizing stars.
