2013MNRAS.431.3003L

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 Hii regions, whose free-free absorption blocks low-frequency radio waves. These Hii 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 Hii regions in starbursts, which is ultimately a cross-section times the density of Hii regions. The cross-sections are calculated by assuming that Hii regions are Strömgren spheres. The coefficient asymptotes to a constant value at low frequencies, because Hii regions partially cover the starburst and are buried part way into the starburst's synchrotron-emitting material. Considering Strömgren spheres around either OB stars or Super Star Clusters, I demonstrate the method by fitting to the low-frequency radio spectrum of M82. I discuss implications of the results for synchrotron spectrum shape, Hii region pressure and free-free emission as a star formation rate indicator. However, these results are preliminary and could be affected by systematics. I argue that there is no volume-filling warm ionized medium in starbursts and that Hii regions may be the most important absorption process down to ∼10MHz. Future data at low and high radio frequency will improve our knowledge of the ionized gas.