The Hot, Accreted Halo of NGC 891

TL;DR

This study measures the metallicity of hot gas around NGC 891, revealing low metallicity gas consistent with accretion from the intergalactic medium and identifying a diffuse nonthermal emission whose origin is uncertain.

Contribution

First measurement of the metallicity of hot halo gas around NGC 891, providing insights into galaxy halo composition and accretion processes.

Findings

Hot gas is dominated by low metallicity near the virial temperature.

Hot gas coexists with hotter gas concentrated near star-forming regions.

A diffuse nonthermal X-ray excess is detected, with an unclear origin.

Abstract

Galaxies are surrounded by halos of hot gas whose mass and origin remain unknown. One of the most challenging properties to measure is the metallicity, which constrains both of these. We present a measurement of the metallicity around NGC 891, a nearby, edge-on, Milky Way analog. We find that the hot gas is dominated by low metallicity gas near the virial temperature at $kT=0.20\pm0.01$ keV and $Z/Z_{\odot} = 0.14\pm0.03$(stat)$^{+0.08}_{-0.02}$(sys), and that this gas co-exists with hotter ($kT=0.71\pm0.04$ keV) gas that is concentrated near the star-forming regions in the disk. Model choices lead to differences of $\Delta Z/Z_{\odot} \sim 0.05$, and higher $S/N$ observations would be limited by systematic error and plasma emission model or abundance ratio choices. The low metallicity gas is consistent with the inner part of an extended halo accreted from the intergalactic medium, which has been modulated by star formation. However, there is much more cold gas than hot gas around NGC 891, which is difficult to explain in either the accretion or supernova-driven outflow scenarios. We also find a diffuse nonthermal excess centered on the galactic center and extending to 5 kpc above the disk with a 0.3-10 keV $L_X = 3.1\times 10^{39}$ erg s$^{-1}$. This emission is inconsistent with inverse Compton scattering or single-population synchrotron emission, and its origin remains unclear.