Flux Contribution and Geometry of the Charge Exchange Emission in the Starburst Galaxy M82

Abstract: Recent X-ray studies of starburst galaxies have found that Charge eXchange (CX) commonly occurs between the outflowing hot plasma and cold gas, possibly from swept-up clouds. However, the total CX flux and the regions where CX occurs have been poorly understood. We present an analysis of the {\it XMM-Newton} observations of M82, a prototype starburst galaxy, aiming to investigate these key properties of the CX emisssion. We have used a blind source separation method in the image analysis with the CCD data which identified a component with the enhanced O-K lines expected from the CX process. Analyzing the RGS spectra from the region identified by the image analysis, we have detected a high forbidden-to-resonance ratio in the \ion{O}{7} He$\alpha$ triplet as well as several emission lines from K-shell transitions of C, N, and O enhanced in the CX process. The CX is less responsible for the emission line of Ne and Mg and the accurate estimation of the CX contribution is confirmed to be crucial in measuring chemical abundances. The temperature of the plasma as electron receiver in the CX process is significantly lower compared to that of the plasma components responsible for most of the X-rays. From the low temperature and an estimation of the CX emitting volume, we find that the CX primarily occurs in a limited region at the interface of the plasma and gas whose temperature rapidly decreases due to thermal conduction.