Revisiting the abundance pattern and charge-exchange emission in the centre of M 82

Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
e-mail: kxfukushima@gmail.com

Received: 22 December 2023
Accepted: 18 March 2024

Abstract

Context. The interstellar medium (ISM) in starburst galaxies contains many chemical elements that are synthesised by core-collapse supernova explosions. By measuring the abundances of these metals, we can study the chemical enrichment within the galaxies and the transportation of metals into the circumgalactic environment through powerful outflows.

Aims. We performed a spectral analysis of the X-ray emissions from the core of M 82 using the Reflection Grating Spectrometer (RGS) on board XMM-Newton to accurately estimate the metal abundances in the ISM.

Methods. We analysed over 300 ks of RGS data observed with 14 position angles, covering a cross-dispersion width of 80 arcsec. We employed multi-temperature thermal plasma components in collisional ionisation equilibrium (CIE) to reproduce the observed spectra, each of which exhibited a different spatial broadening.

Results. The O VII band CCD image shows a broader distribution that those for the O VIII and Fe-L bands. The O VIII line profiles have a prominent double-peaked structure that corresponds to the north- and southward outflows. The O VII triplet feature exhibits marginal peaks. A single CIE component that is convolved with the O VII band image approximately reproduces the spectral shape. A CIE model combined with a charge-exchange emission model also successfully reproduces the O VII line profiles. However, the ratio of these two components varies significantly with the observed position angles, which is physically implausible. Spectral fitting of the broadband spectra suggests a multi-temperature phase in the ISM that is approximated by three components at 0.1, 0.4, and 0.7 keV. Notably, the 0.1 keV component exhibits a broader distribution than the 0.4 and 0.7 keV plasmas. The derived abundance pattern shows super-solar N/O, solar Ne/O and Mg/O, and half-solar Fe/O ratios. These results indicate the chemical enrichment by core-collapse supernovae in starburst galaxies.