Temperature and Metallicity Gradients in the Hot Gas Outflows of M82

TL;DR

This study uses deep X-ray observations to map the properties of hot gas outflows in M82, revealing temperature, density, and metallicity gradients that inform understanding of galactic winds and their interaction with the circumgalactic medium.

Contribution

It provides detailed spatially-resolved measurements of hot plasma properties in M82's superwind, highlighting deviations from simple models and the role of charge exchange and metal enrichment.

Findings

Temperatures and densities peak at the starburst ridge and decrease outward.

Non-thermal emission and charge exchange contribute significantly to X-ray flux.

Metal abundances of O, Ne, Mg, and Fe are constant, while Si and S peak near the center.

Abstract

We utilize deep Chandra X-ray Observatory imaging and spectra of M82, the prototype of a starbursting galaxy with a multiphase wind, to map the hot plasma properties along the minor axis of the galaxy. We extract spectra from 11 regions up to 2.5 kpc from the starbursting midplane and model the data as a multi-temperature, optically thin thermal plasma with contributions from a non-thermal (power-law) component and from charge exchange (CX). We examine the gradients in best-fit parameters, including the intrinsic column density, plasma temperature, metal abundances, and number density of the hot gas as a function of distance from the M82 nucleus. We find that the temperatures and number densities of the warm-hot and hot plasma peak at the starbursting ridge and decreases along the minor axis. The temperature and density profiles are inconsistent with spherical adiabatic expansion of a super-heated wind and suggest mass loading and mixing of the hot phase with colder material. Non-thermal emission is detected in all of the regions considered, and CX comprises 8-25% of the total absorption-corrected, broad-band (0.5-7 keV) X-ray flux. We show that the abundances of O, Ne, Mg, and Fe are roughly constant across the regions considered, while Si and S peak within 500 pc of the central starburst. These findings support a direct connection between the M82 superwind and the warm-hot, metal-rich circumgalactic medium (CGM).