The X-ray surface brightness distribution from diffuse gas

We use simulations to predict the X-ray surface brightness distribution arising from hot, cosmologically distributed diffuse gas. The distribution is computed for two bands, 0.5-2 keV and 0.1-0.4 keV, using a cosmological-constant-dominated cosmology that fits many other observations. We examine a number of numerical issues such as resolution, simulation volume, and pixel size and show that the predicted mean background is sensitive to resolution such that higher resolution systematically increases the mean predicted background. Although this means that we can compute only lower bounds to the predicted level, these bounds are already quite restrictive. Since the observed extragalactic X-ray background is mostly accounted for by compact sources, the amount of the observed background attributable to diffuse gas is tightly constrained. We show that without physical processes in addition to those included in the simulations (such as radiative cooling or nongravitational heating), both bands exceed observational limits. In order to examine the effect of nongravitational heating we explore a simple model of energy injection and show that if this is the only mechanism operating to suppress the background then substantial amounts of heating would be required (i.e., 5 keV per particle when averaged over all baryons). Finally, we also compute the distribution of surface brightness on the sky and show that it has a well-resolved characteristic shape. This shape is substantially modified by non-gravitational heating and can be used as a probe of such energy injection.