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The Spectral Energy Distribution of Dust Emission in the Edge-on Spiral Galaxy NGC 4631 as seen with Spitzer and JCMT

We explore the nature of variations in dust emission within an individual galaxy using 3.6 - 160 micron Spitzer Space Telescope observations and 450 and 850 micron James Clerk Maxwell Telescope observations of the edge-on Sd spiral galaxy NGC 4631 with the goals of understanding the relation between polycyclic aromatic hydrocarbons (PAH) and dust emission, studying the variations in the colors of the dust emission, and searching for possible excess submillimeter emission compared to what is anticipated based on dust models applied to the mid- and far-infrared data. PAH emission at 8 micron is found to correlate best with hot dust emission at 24 micron on kiloparsec scales, although the relation breaks down on scales equal to hundreds of parsecs, possibly because of differences in the mean free paths between the photons that excite the PAHs and heat the dust and possibly because the PAHs are destroyed by the hard radiation fields in the centers of some star formation regions. The ratio of 8 micron PAH emission to 160 micron cool dust emission appears to be a function of radius. The 70/160 and 160/450 micron flux density ratios are remarkably constant even though the surface brightness varies by a factor of 25 in each wave band, which suggests that the emission is from dust heated by a nearly-uniform radiation field. Globally, we find an excess of 850 micron emission relative to what would be predicted by dust models. The 850 micron excess is highest in regions with low 160 micron surface brightness, although the strength and statistical significance of this result 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 micron emission, but we do discuss the other possible mechanisms that could produce the emission.