Is There Still Room for Warm/Hot Gas? Simulating the X-Ray Background Spectrum

At low redshifts, a census of the baryons in all known reservoirs falls a factor of 2-4 below the total baryon density predicted from big bang nucleosynthesis arguments and observed light-element ratios. Recent cosmological hydrodynamic simulations suggest that a significant fraction of these missing baryons could be in the warm/hot intergalactic medium (WHIM) in the filaments and halos within which most field galaxies are embedded. With the release of source count results from Chandra and recent detections of this gas in O VI quasar absorption lines, it becomes interesting to examine the predictions and limits placed on this component of the X-ray background (XRB). We have used new hydrodynamical simulations to predict the total X-ray spectrum from the intergalactic medium and the WHIM alone in the 100 eV-10 keV range. We find that, when uncertainties in the normalization of the observed XRB and the value of Ωb are taken into account, our results are consistent with current observational limits placed on the contribution of emission from the WHIM to the XRB. In the 0.5-2 keV range, we expect the contribution from this component to be 0.24 × 10-12 ergs s-1 cm-2 deg-2 or between 5% and 15% of the extragalactic surface brightness. The peak fraction occurs in the 0.5-0.8 keV range where the predicted line emission mirrors a spectral bump seen in the latest ASCA/ROSAT XRB data.