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