The Spectral Energy Distribution of Dust Emission in the Edge‐on Spiral Galaxy NGC 4631 as Seen with<i>Spitzer</i>and the James Clerk Maxwell Telescope

Abstract

We explore variations in dust emission within the edge-on Sd spiral galaxy NGC 4631 using 3.6Y160 m Spitzer Space Telescope data and 450Y850 m JCMT data with the goals of understanding the relation between PAHs and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is expected from dust models extrapolated from far-infrared wavelengths. The 8 m PAH emission correlates best with 24 m hot dust emission on 1.7 kpc scales, but the relation breaks down on 650 pc scales, possibly because of differences in the mean free paths between photons that excite the PAHs and photons that heat the dust and possibly because the PAHs are destroyed by the hard radiation fields within some star formation regions. The ratio of 8 m PAH emission to 160 m cool dust emission appears to vary as a function of radius. The 70 m/160 m and 160 m /450 m flux density ratios are remarkably constant even though the surface brightnesses vary by factors of 25, which suggests that the emission is from dust heated by a nearly uniform radiation field. Globally, we find an excess of 850 Y1230 m emission relative to what would be predicted by dust models. The 850 m excess is highest in regions with low 160 m surface brightnesses, although the magnitude depends on the model fit to the data. We rule out variable emissivity functions or $4 K dust as the possible origins of this 850 m emission, but we do discuss the other possible mechanisms that could produce the emission.