X-ray emission from haloes of simulated disc galaxies

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 ultraviolet 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 overpredict 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 one-third the solar value this hot gas fraction becomes as low as just a few per cent. We also find that most of the X-ray emission comes from the inner parts (r less than or similar to 20 kpc) of the hot galactic haloes. Finally, we find for realistic choices of the physical parameters that disc galaxy haloes possibly were one order of magnitude brighter in soft X-ray emission at z similar to 1 than at present.