  We investigate the relations among the stellar continuum-subtracted 8 micron
polycyclic aromatic hydrocarbon (PAH 8 micron) emission, 24 micron hot dust
emission, and 160 micron cold dust emission in fifteen nearby face-on spiral
galaxies in the Spitzer Infrared Nearby Galaxies Survey sample. The relation
between PAH 8 and 24 micron emission measured in ~2 kpc regions is found to
exhibit a significant amount of scatter, and strong spatial variations are
observed in the (PAH 8 micron)/24 micron surface brightness ratio. In
particular, the (PAH 8 micron)/24 micron surface brightness ratio is observed
to be high in the diffuse interstellar medium and low in bright star-forming
regions and other locations with high 24 micron surface brightness. PAH 8
micron emission is found to be well-correlated with 160 micron emission on
spatial scales of ~2 kpc, and the (PAH 8 micron)/160 micron surface brightness
ratio is generally observed to increase as the 160 micron surface brightness
increases. These results suggest that the PAHs are associated with the diffuse,
cold dust that produces most of the 160 micron emission in these galaxies, and
the variations in the (PAH 8 micron)/160 micron ratio may generally be
indicative of either the intensity or the spectrum of the interstellar
radiation field that is heating both the PAHs and the diffuse interstellar
dust.
