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 f actor of 2-4 below the total baryon density predicted from big bang nucleos ynthesis arguments and observed light-element ratios. Recent cosmological h ydrodynamic simulations suggest that a significant fraction of these missin g baryons could be in the warm/hot intergalactic medium (WHIM) in the filam ents and halos within which most field galaxies are embedded. With the rele ase of source count results from Chandra and recent detections of this gas in O VI quasar absorption lines, it becomes interesting to examine the pred ictions and limits placed on this component of the X-ray background (XRB). We have used new hydrodynamical simulations to predict the total X-ray spec trum 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 observe d XRB and the value of Omega (b), are taken into account, our results are c onsistent with current observational limits placed on the contribution of e mission from the WHIM to the XRB. In the 0.5-2 keV range, we expect the con tribution from this component to be 0.24 x 10(-12) ergs s(-1) cm(-2) deg(-2 ) or between 5% and 15% of the extragalactic surface brightness. The peak f raction occurs in the 0.5-0.8 keV range where the predicted line emission m irrors a spectral bump seen in the latest ASCA/ROSAT XRB data.