2013ApJ...762...79C

Combining Hα and IRAC images of the nearby spiral galaxy NGC 628, we find that between 30% and 43% of its 8 µm dust emission is not related to recent star formation. Contributions from dust heated by young stars are separated by identifying H II regions in the Hα map and using these areas as a mask to determine the 8 µm dust emission that must be due to heating by older stars. Corrections are made for sub-detection-threshold H II regions, photons escaping from H II regions, and for young stars not directly associated with H II regions (i.e., 10-100 Myr old stars). A simple model confirms that this amount of 8 µm emission can be expected given dust and PAH absorption cross sections, a realistic star formation history, and the observed optical extinction values. A Fourier power spectrum analysis indicates that the 8 µm dust emission is more diffuse than the Hα emission (and similar to observed H I), supporting our analysis that much of the 8 µm-emitting dust is heated by older stars. The 8 µm dust-to-Hα emission ratio declines with galactocentric radius both within and outside of H II regions, probably due to a radial increase in disk transparency. In the course of this work, we have also found that intrinsic diffuse Hα fractions may be lower than previously thought in galaxies, if the differential extinction between H II regions and diffuse regions is taken into account.