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 of 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 Z ⊙ 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 the X‐ray Evolving Universe Spectroscopy mission ( XEUS ) should be able to single out the halo emission of highly inclined Milky Way‐sized disc galaxies out to 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.