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