  We examine the relation between polycyclic aromatic hydrocarbon (PAH)
emission at 8 microns and far-infrared emission from hot dust grains at 24
microns and from large dust grains at 160 and 250 microns in the nearby spiral
galaxies NGC 2403 and M83 using data from the Spitzer Space Telescope and
Herschel Space Observatory. We find that the PAH emission in NGC 2403 is better
correlated with emission at 250 microns from dust heated by the diffuse
interstellar radiation field (ISRF) and that the 8/250 micron surface
brightness ratio is well-correlated with the stellar surface brightness as
measured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixed
with cold large dust grains in the diffuse interstellar medium (ISM) and that
the PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears more
strongly correlated with 160 micron emission originating from large dust grains
heated by star forming regions. However, the PAH emission in M83 is low where
the 24 micron emission peaks within star forming regions, and enhancements in
the 8/160 micron surface brightness ratios appear offset relative to the dust
and the star forming regions within the spiral arms. This suggests that the
PAHs observed in the 8 micron band are not excited locally within star forming
regions but either by light escaping non-axisymmetrically from star forming
regions or locally by young, non-photoionising stars that have migrated
downstream from the spiral density waves. The results from just these two
galaxies show that PAHs may be excited by different stellar populations in
different spiral galaxies.
