QUANTIFYING NON-STAR-FORMATION-ASSOCIATED 8 μm DUST EMISSION IN NGC 628

Combining H alpha and IRAC images of the nearby spiral galaxy NGC 628, we find that between 30% and 43% of its 8 mu m 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 H alpha map and using these areas as a mask to determine the 8 mu m 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 mu m 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 mu m dust emission is more diffuse than the II alpha emission (and similar to observed H I), supporting our analysis that much of the 8 mu m-emitting dust is heated by older stars. The 8 mu m dust-to-H alpha 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 H alpha fractions may be lower than previously thought in galaxies, if the differential extinction between H II regions and diffuse regions is taken into account.