Diffuse X-Ray Emission in M51: A Hierarchical Bayesian Spatially Resolved Spectral Analysis

Abstract X-ray observations can be used to effectively probe the galactic ecosystem, particularly its hot and energetic components. However, existing X-ray studies of nearby star-forming galaxies are limited by insufficient data statistics and a lack of suitable spectral modeling to account for X-ray emission and absorption geometry. We present results from an X-ray spectral study of M51 using 1.3 Ms Chandra data, the most extensive for such a galaxy. This allows the extraction of diffuse X-ray emission spectra from phase-dependent regions of spiral arms using a logarithmic spiral coordinate system. A hierarchical Bayesian approach analyzes these spectra, testing models from simple single-temperature hot plasma to those including distributed hot plasma and X-ray-absorbing cool gas. We recommend a model that fits the spectra well, featuring a galactic corona with a lognormal temperature distribution and a disk with mixed X-ray emissions and absorption. In this model, only half of the coronal emission is subject to internal absorption. The best-fit column density of absorbing gas is roughly twice that inferred from optical extinction of stellar light. The temperature distribution shows a mean temperature of ∼0.1 keV and an average one-dex dispersion that is enhanced on the spiral arms. The corona’s radiative cooling might balance the mechanical energy input from stellar feedback. These results highlight the effectiveness of X-ray mapping of the corona and cool gas in spiral galaxies.