  Bolometric and 0.2-2 keV X-ray luminosities of the hot gas haloes of
simulated disc galaxies have been calculated at redshift z=0. The TreeSPH
simulations are fully cosmological and the sample of 44 disc galaxies span a
range in characteristic circular speeds of V_c = 130-325 km/s. The galaxies
have been obtained in simulations with a considerable range of physical
parameters, varying the baryonic fraction, the gas metallicity, the
meta-galactic UV field, the cosmology, the dark matter type, and also the
numerical resolution. The models are found to be in agreement with the (few)
relevant X-ray observations available at present. The amount of hot gas in the
haloes is also consistent with constraints from pulsar dispersion measures in
the Milky Way. Forthcoming XMM and Chandra observations should enable much more
stringent tests and provide constraints on the physical parameters. We find
that simple cooling flow models over-predict X-ray luminosities by up to two
orders of magnitude for high (but still realistic) cooling efficiencies
relative to the models presented here. Our results display a clear trend that
increasing cooling efficiency leads to decreasing X-ray luminosities at z=0.
The reason is found to be that increased cooling efficiency leads to a
decreased fraction of hot gas relative to total baryonic mass inside of the
virial radius at present. At gas metal abundances of a third solar this hot gas
fraction becomes as low as just a few percent. We also find that most of the
X-ray emission comes from the inner parts (inner about 20 kpc) of the hot
galactic haloes. Finally, we find for realistic choices of the physical
parameters that disc galaxy haloes possibly were more than one order of
magnitude brighter in soft X-ray emission at z=1, than at present.
