NASA NTRS1990
A few galaxies have been observed at submillimeter wavelengths (Stark et al., 1988; Eales, Wynn-Williams, and Duncan, 1988). These observations do not support the hypothesis that there exist substantial amounts of cool dust generating submillimeter radiation in excess of predictions from modified blackbody curves of the form (1/lambda)exp 1.5 B sub v (v, T) fit to total 100 and 160 micron fluxes. However, researchers find relatively cold dust (approx. 17 to 23 degrees K) at several interarm locations in the central 5 minute by 5 minute region of NGC 6946 Sc(s)II. Since molecular gas is the predominate constituent of the interstellar medium (ISM) in this region, such low dust temperatures are indicative of quiescent molecular clouds and low star formation rates. Based on the contribution of cool grains to the 160 micron surface brightness distribution, regions of low star formation activity can be identified. A 1/lambda exp 1.5 emissivity law is assumed for this analysis. Researchers assumed a two temperature grain model in an effort to decompose the far infrared surface brightness distribution of NGC 6946 into contributions from cool, quiescent and active, star forming regions. This model has some validity; although an oversimplification of the dust energetics, researchers find from it that, where giant HII regions are sparse, a significant fraction of the 160 micron emission is emitted by a cool grain component which the Infrared Astronomy Satellite (IRAS) could barely detect.