Far-infrared radiation from dust heated by the interstellar radiation field in spiral galaxies

We calculate the 60 and 100 mu m radiation emitted by dust in H I gas in spiral galaxies. The dust is assumed to be heated by the general interstellar radiation field. We base our results on the dust model by Desert et al., who have calculated the expected emission in the IRAS bands from dust heated by radiation fields with the same spectral shape as those observed in the solar neighborhood but of varying intensity. We estimate the amount of dust in a galaxy by assuming that the dust column density is proportional to that of H I at each radius. The intensity of the radiation field is derived from the observed blue-light profiles of the galaxies, corrected for extinction and projection effects. By combining these two observables with the dust heating model, we are able to show that the interstellar radiation field can account for a significant fraction of the far-infrared (FIR) emission in most spirals, For the best resolved galaxies, M31 and M33, the radial 60 and 100 mu m profiles predicted from our model agree well with the observed profiles. For nine other resolved spirals, the overall shape of the modeled and observed FIR radial profiles are also similar. Overall, for a sample of 20 spirals, the 60 and 100 mu m fluxes derived from our model are, on average, about half of the observed fluxes. We also compare the predicted and observed 12 and 25 mu m fluxes and find good consistency. The analysis supports a picture where the 60 and 100 um emission from spirals is a combination of two components, relatively cool (20-25 K) dust heated by the interstellar radiation field and warmer dust heated by OB stars. Which of these two dominates may vary from galaxy to galaxy. The agreement at shorter wavelengths argues for the likely existence of very small grains in spiral galaxies. Unless our understanding of dust models and the explanation for infrared emission from Galactic cirrus are incorrect, it appears that the role of the interstellar radiation field in heating dust in spiral galaxies is important enough that its contribution should not be ignored.