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 4 \times 10 ^ { 39 } erg s ^ { -1 } .
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 \sim 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 .