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The debate about the origin of the 60- and 100-micron emission from spiral galaxies continues. Do only OB stars heat the dust to generate the luminosities observed by IRAS, or can dust heated by an older stellar population contribute significantly as well? Only in the first case do the infrared fluxes provide a direct link to the rate of formation of massive stars. We have developed a method for calculating how much dust heated by the interstellar radiation field can contribute to the 60- and 100-micron emission of particular spiral galaxies. We assume that the observed B light profile of a spiral galaxy traces the interstellar radiation field, and the observed HI distribution the dust. We convert the observed B profile for individual galaxies to the intensity that a dust grain in the disk sees, taking into account the effects of disk inclination, and observed dust column. We then use the well-established dust model by Des\`ert, Boulanger, and Puget to determine the emission in the IRAS bands.
Our program calculates 60- and 100-micron radial profiles and total fluxes for the galaxies in our sample. The radial 60- and 100-micron profiles agree very well with the observed profiles for M31 and M33, without the need for radial dust-to-gas gradients. In addition, the interstellar radiation field is capable of contributing a significant fraction of the observed 60- and 100-micron emission for most spiral galaxies in our sample. As expected, the calculated dust temperatures are typically lower than the observed ones, in agreement with the notion that in most spiral galaxies there will be a contribution from warm dust heated by OB stars as well. Finally, we find that the galaxies for which our predicted fluxes are low, are exactly those which are known to have very high massive star formation rates. The results support a picture where the IRAS 60- and 100-micron emission is due to two components, heated by different stellar populations, not just to dust heated by O and B stars.
We acknowledge support from NSF (AST9123777), NASA (NAG5-2426), and the NMSU Spacegrant Consortium.
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