  Models of disk galaxy formation commonly predict the existence of an extended
reservoir of hot gas surrounding massive spirals at low redshift. As a test of
these models, we have obtained X-ray and optical data of the two massive
edge-on spirals NGC 5746 and NGC 5170, in order to investigate the amount and
origin of hot gas in their disks and halos. Chandra observations of NGC 5746
reveal evidence for diffuse X-ray emission with a total luminosity of ~7 x
10^39 erg/s surrounding this galaxy out to at least ~20 kpc from the disk,
whereas an identical study of the less massive NGC 5170 fails to detect any
extraplanar X-ray emission. Unlike the case for other disk galaxies with
detected X-ray halos, the halo emission around NGC 5746 is not accompanied by
extraplanar H-alpha or radio emission, and there is no evidence for significant
nuclear or starburst activity in the disk. In contrast to these other cases,
the emission around NGC 5746 therefore appears to arise from the cooling of
externally accreted material rather than from disk outflows. To verify this
idea, we present results of cosmological simulations of galaxy formation and
evolution, showing our observations to be in good agreement with expectations
for cosmological accretion, while also confirming that the X-ray halos of other
spirals do not fit well into an accretion scenario. We find that the estimated
cooling rate of hot halo gas around NGC 5746 would provide sufficient material
for star formation in the disk to proceed at its present rate. This lends
support to the idea that a supply of hot ambient gas is potentially available
as fuel for star formation in massive, nearby spirals, and suggests that
accretion of hot gas could be important for maintaining the stellar disks of
such galaxies.
