CO+ in M 82: A Consequence of Irradiation by X-rays
Abstract
Based on its strong CO+ emission it is argued that the M 82 star-burst galaxy is exposed to a combination of FUV and X-ray radiation. The latter is likely to be the result of the star-burst superwind, which leads to diffuse thermal emission at ~0.7 keV, and a compact hard, 2-10 keV, source (but not an AGN). Although a photon-dominated region (FUV) component is clearly present in the nucleus of M 82, and capable of forming CO+, only X-ray irradiated gas of density 103-105 cm-3 can reproduce the large, ~(1-4)x1013 cm-2, columns of CO+ that are observed toward the proto-typical star-burst M 82. The total X-ray luminosity produced by M 82 is weak, ~1041 erg s-1, but this is sufficient to drive the formation of CO+.
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