  We present an X-ray study of the massive edge-on Sa galaxy, Sombrero (M~104;
NGC 4594), based on XMM-Newton and Chandra observations. A list of 62
XMM-Newton and 175 Chandra discrete X-ray sources is provided. At energies
above 2 keV, the source-subtracted X-ray emission is distributed similarly as
the stellar K-band light and is primarily due to the residual emission from
discrete sources. At lower energies, however, a substantial fraction of the
source-subtracted emission arises from diffuse hot gas extending to ~20 kpc
from the galactic center. The observed diffuse X-ray emission from the galaxy
shows a steep spectrum that can be characterized by an optically-thin thermal
plasma with temperatures of ~0.6-0.7 keV, varying little with radius. The
diffuse emission has a total luminosity of ~3e39 erg/s in the 0.2-2 keV energy
range. This luminosity is significantly smaller than the prediction by current
numerical simulations for galaxies as massive as Sombrero. We argue that the
stellar feedback likely plays an essential role in regulating the physical
properties of hot gas. Indeed, the observed diffuse X-ray luminosity of
Sombrero accounts for at most a few percent of the expected mechanical energy
input from Type Ia supernovae. The inferred gas mass and metal content are also
substantially less than those expected from stellar ejecta. We speculate that a
galactic bulge wind, powered primarily by Type Ia supernovae, has removed much
of the 'missing' energy and metal-enriched gas from regions revealed by the
X-ray observations.
