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We explore the nature of variations in dust emission within an individualgalaxy using 3.6 - 160 micron Spitzer Space Telescope observations and 450 and850 micron James Clerk Maxwell Telescope observations of the edge-on Sd spiralgalaxy NGC 4631 with the goals of understanding the relation between polycyclicaromatic hydrocarbons (PAH) and dust emission, studying the variations in thecolors of the dust emission, and searching for possible excess submillimeteremission compared to what is anticipated based on dust models applied to themid- and far-infrared data. PAH emission at 8 micron is found to correlate bestwith hot dust emission at 24 micron on kiloparsec scales, although the relationbreaks down on scales equal to hundreds of parsecs, possibly because ofdifferences in the mean free paths between the photons that excite the PAHs andheat the dust and possibly because the PAHs are destroyed by the hard radiationfields in the centers of some star formation regions. The ratio of 8 micron PAHemission 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 eventhough the surface brightness varies by a factor of 25 in each wave band, whichsuggests that the emission is from dust heated by a nearly-uniform radiationfield. Globally, we find an excess of 850 micron emission relative to whatwould be predicted by dust models. The 850 micron excess is highest in regionswith low 160 micron surface brightness, although the strength and statisticalsignificance of this result depends on the model fit to the data. We rule outvariable emissivity functions or ~4 K dust as the possible origins of this 850micron emission, but we do discuss the other possible mechanisms that couldproduce the emission.Comment: Accepted to the Astrophysical Journa

astrophysical journal/the astrophysical journal

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

The Spectral Energy Distribution of Dust Emission in the Edge‐on Spiral Galaxy NGC 4631 as Seen withSpitzerand the James Clerk Maxwell Telescope