SPECTRAL MODELING OF THE CHARGE-EXCHANGE X-RAY EMISSION FROM M82

It has been proposed that the charge-exchange (CX) process at the interface between hot and cool interstellar gases could contribute significantly to the observed soft X-ray emission in star-forming galaxies. We analyze the XMM-Newton/reflection grating spectrometer (RGS) spectrum of M82 using a newly developed CX model combined with a single-temperature thermal plasma to characterize the volume-filling hot gas. The CX process is largely responsible for not only the strongly enhanced forbidden lines of the Ka triplets of various He-like ions but also good fractions of the Ly alpha transitions of C (similar to 87%), 0 VIII, and N VII (>= 50%) as well. In total about a quarter of the X-ray flux in the RGS 6-30 angstrom band originates in the CX. We infer an ion incident rate of 3 x 10(51) s(-1) undergoing CX at the hot and cool gas interface and an effective area of the interface of 2 x 1045 cm2 that is one order of magnitude larger than the cross section of the global biconic outflow. With the CX contribution accounted for, the best-fit temperature of the hot gas is 0.6 keV, and the metal abundances are approximately solar. We further show that the same CX/thermal plasma model also gives an excellent description of the EPIC-pn spectrum of the outflow Cap, projected at 11.6 kpc away from the galactic disk of M82. This analysis demonstrates that the CX is potentially an important contributor to the X-ray emission from starburst galaxies and also an invaluable tool to probe the interface astrophysics.