CO+ in M 82: A Consequence of Irradiation by X-rays

TL;DR

The paper demonstrates that the strong CO+ emission in M 82 is primarily caused by X-ray irradiation from starburst activity, rather than solely by FUV radiation, highlighting the role of X-rays in molecular chemistry in starburst galaxies.

Contribution

It shows that X-ray irradiation, rather than FUV radiation alone, can account for the observed CO+ columns in M 82, emphasizing the importance of X-ray processes in starburst environments.

Findings

X-ray irradiation explains CO+ abundance in M 82.

FUV radiation alone cannot produce observed CO+ levels.

Weak X-ray luminosity suffices for CO+ formation.

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 10^3-10^5 cm^-3 can reproduce the large, ~(1-4)x10^13 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, ~10^41 erg s^-1, but this is sufficient to drive the formation of CO+.