Are Spiral Galaxies Optically Thick?
We have re-examined the classical optical evidence for the traditional view that spiral discs are optically thin and find it to be highly model dependent and unconvincing. We find that the data can be equally well fitted by discs with a physically thin but optically thick dust layer. An optically thick model is consistent with the near infra-red and IRAS observations, with the surface brightness, HI column densities, CO intensities and with H03B1 measurements. Optically thick galaxies could have up to 2 magnitudes of extinction in the blue, and the stirring upwards of the dust could account for the most luminous IRAS galaxies without recourse to massive bursts of star formation. If disc galaxies are optically thick then this would affect the observed mass-to-light ratio and lead to obscuration of background QSOs.
KeywordsOptical Depth Star Formation Column Density Spiral Galaxy Surface Brightness
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