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Evidence against a simple two-component model for the far-infrared emission from galaxies

Eales, Stephen Anthony and Devereux, Nicholas A. 1990. Evidence against a simple two-component model for the far-infrared emission from galaxies. Presented at: Second Wyoming Conference on the Interstellar Medium in External. Galaxies, Grand Teton National Park, WY, USA, 3-7 July 1989. Published in: Hollenbach, David J. and Thronson, Harley A. eds. The Interstellar Medium in External Galaxies: Summaries of Contributed Papers. NASA conference publication , vol. 3084. Washington DC: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Division, pp. 103-104.

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Abstract

Two of the first Infrared Astronomy Satellite (IRAS) results were that galaxies have a wide range of values for the ratio of 60 micron to 100 micron flux density (0.2 less than or equal to S60/S sub 100 less than or equal to 1.0) and that this ratio is correlated with Lfir, Lb, Lfir being the total far-infrared luminosity and Lb being the luminosity at visible wavelengths (de Jong et al. 1984; Soifer et al. 1984). From these results arose the following simple model for the far-infrared emission from galaxies (de Jong et al. 1984), which has remained the standard model ever since. In this model, the far-infrared emission comes from two dust components: warm dust (T approx. equals 50 K) intermingled with, and heated by, young massive OB stars in molecular clouds and HII regions, and colder dust (T approx. equals 20 K) associated with the diffuse atomic hydrogen in the interstellar medium and heated by the general interstellar radiation field. As the number of young stars in a galaxy increases, S60/S sub 100 increases, because there is a greater proportion of warm dust, and so does Lfir/L sub b, because most of the radiation from the young stars is absorbed by the dust, leading to a swifter increase in far-infrared emission than in visible light. Although this model explains the basic IRAS results, it is inelegant. It uses two free parameters to fit two data (the 60 and 100 micron flux densities)-and there are now several observations that contradict it. Despite these major problems with the two-component model, it is not clear what should be put in its place. When considering possible models for the far-infrared emission from galaxies, the observational evidence for our own galaxy must be considered. Researchers suspect that the study by Boulanger and Perault (1988) of the far-infrared properties of the local interstellar medium may be particularly relevant. They showed that molecular clouds are leaky - that most of the light from OB stars in molecular clouds does not heat the dust in the clouds, but instead leaks out. The consequence of this is that that while most of the far-infrared emission from the solar neighborhood is from dust associated with diffuse HI, this dust is mostly heated by young stars.

Item Type: Conference or Workshop Item (Paper)
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Publisher: National Aeronautics and Space Administration, Office of Management, Scientific and Technical Information Division
Last Modified: 04 Jun 2017 05:11
URI: http://orca-mwe.cf.ac.uk/id/eprint/49433

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