解释星暴星系团的低频射电谱：Strömgren球的布丁
Interpreting the Low Frequency Radio Spectra of Starburst Galaxies: A Pudding of Strömgren Spheres
摘要：
星暴星系的低频无线电辐射是有用的，但它可以通过几种方式被吸收。最重要的是，星暴星系是许多H II区的家园，它们的自由吸收阻挡了低频无线电波。这些H II区域是离散的天体，但大多数多波长的星爆模型假设电离气体的均匀介质，如果它们包括吸收的话。我计算了星爆中H II区的有效吸收系数，它最终是一个截面乘以H II区的密度。通过假设H II区是Strömgren球来计算截面。该系数在低频率下渐近于一个恒定值，因为H II区域部分覆盖了星暴，并且部分被埋在星暴的同步辐射物质中。考虑到无论是OB星或超级星团周围的Strömgren球体，我证明了该方法通过拟合M82的低频无线电频谱。我讨论的同步辐射光谱形状，H II区域的压力，和自由的自由发射作为恒星形成率指标的结果的影响。然而，这些结果是初步的，并可能受到系统学的影响。我认为，有没有体积填充温暖的电离介质中的星爆，和H II区域可能是最重要的吸收过程下降到~10 MHz。未来在低和高射频的数据将提高我们对电离气体的认识。
英文摘要：
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 blocks 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 sections are calculated by assuming that H II regions are Strömgren 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 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, H II 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 H II regions may be the most important absorption process down to ~10 MHz. Future data at low and high radio frequency will improve our knowledge of the ionized gas.
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