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For refcode 1993AJ....106..948D:
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1993AJ....106..948D THE ORIGIN OF THE FAR INFRARED LUMINOSITY WITHIN THE SPIRAL GALAXY NGC 6946 NICHOLAS A. DEVEREUX Department of Astronomy, New Mexico State University, Las Cruces, New Mexico 88001 Electronic mail: devereux@nmsu.edu JUDITH S. YOUNG Department of Physics & Astronomy, and the Five College Radio Astronomy Observatory, University of Massachusetts, Amherst, Massachusetts 01003 Electronic mail: young@fcraol.phast.umass.edu Received 1993 March 10; revised 1993 May 15 ABSTRACT High resolution, 45", measurements of the 160 micron and H{alpha} luminosity have been examined at 158 locations within the spiral galaxy NGC 6946 in order to elucidate the origin of the far infrared luminosity. After correcting the H{alpha} luminosity for extinction and extrapolating the 160 micron luminosity to a 40-1000 micron far infrared luminosity we find that the far infrared luminosity is comparable to that expected from the O and B stars which are required to ionize the hydrogen gas. The results show that the far infrared luminosity is in quantitative agreement with that expected from O and B stars at all locations within NGC 6946. Further, the IRAS HiRes 60 and 100 micron images of NGC 6946 indicate that the temperature of the dust primarily responsible for the far infrared luminosity is ~33 K ({lambda}^-1^) and similar to that expected for H II regions over the entire star-forming disk. The mass of dust required to radiate the 60 and 100 micron emission, measured for NGC 6946 by IRAS, is about a factor of 10 lower than the dust content of NGC 6946 estimated from the atomic and molecular gas masses and a Galactic gas/dust mass ratio. The most plausible explanation for the discrepancy is that the majority (~90%) of the dust within NGC 6946 is radiating at wavelengths longward of 100 microns, and must therefore be cooler than the ~33 K dust that dominates the emission measured by IRAS. The 170 and 160 micron measurements of NGC 6946, obtained by Smith (1984) and Engargiola [ApJ, 76,875(1991)], provide independent support for large quantities of cold dust, as both the 160 and 170 micron flux measurements are significantly higher than would be expected if all the dust in NGC 6946 were radiating at the same temperature as that measured by IRAS. The mass of warm dust is used to constrain the mass of dust that radiates at wavelengths longward of 100 microns, and the shape of the far infrared spectral energy distribution is used to constrain the temperature of the dust. The observations require the bulk, ~90%, of the dust mass within NGC 6946 to be cold with a temperature < 17 K. It has long been known that molecular gas dominates the interstellar medium within NGC 6946 and the temperature derived for the dust is consistent with that expected for molecular clouds. The far infrared luminosity of NGC 6946 is dominated by thermal emission from warm, ~33 K, dust even though most (~90%) of the dust mass within NGC 6946 is much colder with a temperature of ~17 K. The ability for a small mass of warm dust to dominate the luminosity of significantly greater quantities of colder dust arises as a natural consequence of the strong temperature dependence of thermal emission.
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