High-redshift X-ray properties of the haloes of simulated disc galaxies

X-ray luminosities and surface brightness profiles of the hot gas haloes of simulated disc galaxies at redshifts z=0-2 are presented. The galaxies are extracted from fully cosmological simulations and correspond in mass to the Milky Way. We find that the bolometric X-ray luminosities of the haloes decrease by a factor 4-10 from z~1 to z~0, reflecting the decrease in the rate at which hot halo gas cools out on to the disc. At all redshifts, most of the emission is found to originate within 10-15 kpc of the disc. When combined with models in which the evolution of disc X-ray luminosity is dominated by X-ray binaries, the predicted halo luminosities at z~1 show good agreement with constraints from spiral galaxies in Chandra Deep Field data. There is an indication that haloes with a metal abundance of 0.3 solar overpredict observed X-ray luminosities at z~1, suggesting that halo metallicities are lower than this value. Prospects for direct detection of the haloes of Milky Way-sized galaxies with current and future X-ray instrumentation are discussed. It is found that XEUS should be able to single out the halo emission of highly inclined Milky Way-sized disc galaxies out to approximately z=0.3. For such galaxies in this redshift interval, we estimate a lower limit to the surface density of detectable haloes on the sky of ~10 deg^(-2). More generally, owing to their luminosity evolution, the optimum redshifts at which to observe such haloes could be 0.5<z<1, depending on their assembly history.