The relations among 8, 24 and 160 μm dust emission within nearby spiral galaxies

We investigate the relations among the stellar continuum-subtracted 8 mu m polycyclic aromatic hydrocarbon (PAH 8 mu m) emission, 24 mu m hot dust emission and 160 mu m cold dust emission in 15 nearby face-on spiral galaxies in the Spitzer Infrared Nearby Galaxies Survey sample. The relation between PAH 8 and 24 mu m emission measured in similar to 2 kpc regions is found to exhibit a significant amount of scatter, and strong spatial variations are observed in the (PAH 8 mu m)/24 mu m surface brightness ratio. In particular, the (PAH 8 mu m)/24 mu m 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 mu m surface brightness. PAH 8 mu m emission is found to be well-correlated with 160 mu m emission on spatial scales of similar to 2 kpc, and the (PAH 8 mu m)/160 mu m surface brightness ratio is generally observed to increase as the 160 mu m surface brightness increases. These results suggest that the PAHs are associated with the diffuse, cold dust that produces most of the 160 mu m emission in these galaxies, and the variations in the (PAH 8 mu m)/160 mu m 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.