  The existence of X-ray luminous gaseous coronae around massive disc galaxies
is a long-standing prediction of galaxy formation theory in the cold dark
matter cosmogony. This prediction has garnered little observational support,
with non-detections commonplace and detections for only a relatively small
number of galaxies which are much less luminous than expected. We investigate
the coronal properties of a large sample of bright, disc-dominated galaxies
extracted from the GIMIC suite of cosmological hydrodynamic simulations
recently presented by Crain et al. Remarkably, the simulations reproduce the
observed scalings of X-ray luminosity with K-band luminosity and star formation
rate and, when account is taken of the density structure of the halo, with disc
rotation velocity as well. Most of the star formation in the simulated galaxies
(which have realistic stellar mass fractions) is fuelled by gas cooling from a
quasi-hydrostatic hot corona. However, these coronae are more diffuse, and of a
lower luminosity, than predicted by the analytic models of White & Frenk
because of a substantial increase in entropy at z ~ 1-3. Both the removal of
low entropy gas by star formation and energy injection from supernovae
contribute to this increase in entropy, but the latter is dominant for halo
masses M_200 <~ 10^(12.5) Msun. Only a small fraction of the mass of the hot
gas is outflowing as a wind but, because of its high density and metallicity,
it contributes disproportionally to the X-ray emission. The bulk of the X-ray
emission, however, comes from the diffuse quasi-hydrostatic corona which
supplies the fuel for ongoing star formation in discs today. Future deep X-ray
observations with high spectral resolution (e.g. with NeXT/ASTRO-H or IXO)
should be able to map the velocity structure of the hot gas and test this
fundamental prediction of current galaxy formation theory.
