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

We investigate the relations among the stellar continuum-subtracted 8 µm polycyclic aromatic hydrocarbon (PAH 8 µm) emission, 24 µm hot dust emission and 160 µm 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 µm 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 µm)/24 µm surface brightness ratio. In particular, the (PAH 8 µm)/24 µm 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 µm surface brightness. PAH 8 µm emission is found to be well-correlated with 160 µm emission on spatial scales of ~2 kpc, and the (PAH 8 µm)/160 µm surface brightness ratio is generally observed to increase as the 160 µm surface brightness increases. These results suggest that the PAHs are associated with the diffuse, cold dust that produces most of the 160 µm emission in these galaxies, and the variations in the (PAH 8 µm)/160 µm 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.