Hot gas halos around disk galaxies: Confronting cosmological simulations
  with observations
  Models of disk galaxy formation commonly predict the existence of an extended
reservoir of accreted hot gas surrounding massive spirals at low redshift. As a
test of these models, we use X-ray and H-alpha data of the two massive,
quiescent edge-on spirals NGC 5746 and NGC 5170 to investigate the amount and
origin of any hot gas in their halos. Contrary to our earlier claim, the
Chandra analysis of NGC 5746, employing more recent calibration data, does not
reveal any significant evidence for diffuse X-ray emission outside the optical
disk, with a 3-sigma upper limit to the halo X-ray luminosity of 4e39 erg/s. An
identical study of the less massive NGC 5170 also fails to detect any
extraplanar X-ray emission. By extracting hot halo properties of disk galaxies
formed in cosmological hydrodynamical simulations, we compare these results to
expectations for cosmological accretion of hot gas by spirals. For Milky
Way-sized galaxies, these high-resolution simulations predict hot halo X-ray
luminosities which are lower by a factor of ~2 compared to our earlier results
reported by Toft et al. (2002). We find the new simulation predictions to be
consistent with our observational constraints for both NGC 5746 and NGC 5170,
while also confirming that the hot gas detected so far around more actively
star-forming spirals is in general probably associated with stellar activity in
the disk. Observational results on quiescent disk galaxies at the high-mass end
are nevertheless providing powerful constraints on theoretical predictions, and
hence on the assumed input physics in numerical studies of disk galaxy
formation and evolution.
