1206.7100 (Brian C. Lacki)
Brian C. Lacki
The low frequency radio emission of starburst galaxies is informative, but it can be absorbed in several ways. Most importantly, starburst galaxies are home to many H II regions, whose free-free absorption can obscure low frequency radio waves. These H II regions are discrete objects, but most multiwavelength models of starbursts assume a uniform medium of ionized gas, if they include the absorption at all. I calculate the effective absorption coefficient of H II regions in starbursts, which is ultimately a cross section times the density of H II regions. The cross section can be easily calculated by assuming that H II regions are Str\"omgren spheres. The coefficient asymptotes to a constant value at low frequencies, because H II regions partially cover the starburst, and are buried part way into the starburst's synchrotron emitting material. Considering Str\"omgren spheres around both O stars and Super Star Clusters, I apply the calculations to the low frequency radio spectrum of M82. Far from being opaque at low frequencies, I find that M82 mostly transmits its radio flux. I also find that starbursts are transparent down to a few MHz to other possible absorption processes, such as free-free absorption from the diffuse superwind phase, synchrotron self-absorption, and the Razin effect. Hence, starburst galaxies should be observable with new low frequency radio telescopes.
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http://arxiv.org/abs/1206.7100
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