Revisiting the abundance pattern and charge-exchange emission in the M82 centre

The interstellar medium (ISM) in starburst galaxies contains plenty of chemical elements synthesised by core-collapse supernova explosions. By measuring the abundances of these metals, we can study the chemical enrichment within galaxies and the transportation of metals into circumgalactic environments through powerful outflows. We perform the spectral analysis of the X-ray emissions from the M82 core using the Reflection Grating Spectrometer (RGS) onboard XMM-Newton to accurately estimate the metal abundances in the ISM. We analyse over 300 ks of RGS data observed with fourteen position angles, covering an 80 arcsec cross-dispersion width. We employ multi-temperature thermal plasma components in collisional ionisation equilibrium (CIE) to reproduce the observed spectra, each exhibiting different spatial broadenings. The O vii band CCD image shows a broader distribution compared to those for O viii and Fe-L bands. The O viii line profiles have a prominent double-peaked structure, corresponding to the northward and southward outflows. The O vii triplet feature exhibits marginal peaks, and a single CIE component, convolved with the O vii band image, approximately reproduces the spectral shape. Combining a CIE model 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, 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 enrichments by core-collapse supernovae in starburst galaxies.

K. Fukushima, S. B. Kobayashi, K. Matsushita · 2024