X-ray emission from the Sombrero galaxy: a galactic-scale outflow

TL;DR

This study uses Chandra X-ray observations to analyze the diffuse hot gas in the Sombrero galaxy, revealing a galactic-scale outflow that impacts the galaxy's energy and mass balance, challenging existing models.

Contribution

It provides detailed measurements of hot gas properties and demonstrates the presence of a galactic outflow that explains missing energy and mass in the galaxy.

Findings

Diffuse hot gas extends beyond stellar light, indicating a galactic outflow.

Measured gas temperature is ~0.6 keV, with lower temperatures along the disk.

Observed energy and mass inputs are not fully accounted for by supernovae and stellar evolution.

Abstract

Based on new and archival Chandra observations of the Sombrero galaxy (M 104), we study the diffuse X-ray emission in and around its massive stellar bulge. The 2-6 keV unresolved emission from the bulge region closely follows the K-band star light and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the starlight, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity is recently established. We find a gas temperature of ~0.6 keV with little variation across the field of view, except for a lower temperature of ~0.3 keV along the stellar disk. We measure a total intrinsic 0.3-2 keV luminosity of ~2e39 erg/s, which is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating source for the diffuse gas. We also show that neither the expected energy released by Type Ia supernovae nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continues to pose challenges to theoretical studies.