Hydrodynamic Simulation of the Cosmological X‐Ray Background

(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predictthe extragalactic X-ray background (XRB), focussing on emission from theintergalactic medium (IGM). We also include X-rays from point sourcesassociated with galaxies in the simulation, and make maps of the angulardistribution of the emission. We find that filaments in the maps are notevident, being diluted by projection. In the soft (0.5-2 keV) band, the meanintensity of radiation from intergalactic and cluster gas is 2.3*10^-12ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible atthe ~1 sigma level with estimates of the unresolved soft background from ROSATand {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated withthe IGM. Relative to AGN flux, the IGM component peaks at a lower redshift(median z~0.45) so its clustering makes an important contribution to that ofthe total XRB. The angular correlations on 0.1-10 arcmin scales aresignificant, with an amplitude roughly consistent with an extrapolation ofrecent ROSAT results to small scales. A cross-correlation of the XRB againstnearby galaxies taken from a simulated redshift survey also yields a strongsignal from the IGM. Although some recent papers have argued that the expectedsoft band intensity from gas in galaxy, group, and cluster halos would exceedXRB limits unless much of the gas is expelled by supernova feedback, we obtainreasonable compatibility with current observations in a simulation thatincorporates cooling, star formation, and only modest feedback. A prediction ofour model is that the unresolved portion of the soft XRB will remain mostlyunresolved.