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 ^ { -1 } .
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 ( r \la 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 \sim 1 , than at present .