Primeval gas clouds and the low-energy X-ray background

The origin of the diffuse X-ray background radiation has not yet been satisfactorily explained. Several authors have estimated the contributions from distant unresolved counterparts of known extragalactic X-ray sources; they do not explain more than 20%--40% of the total observed background. In this paper we consider a possible origin of the low-energy (E9 or approx. =3 keV) background as due to fragmenting clouds of hot gas: the collapsing protoclusters forming in the early universe. We assume that these clouds have a range of densities and sizes similar to those of clusters of galaxies. Under a maximal heating model in which some of these clouds are heated up to a temperature such that their thermal pressure stabilizes them against gravitational collapse, we find a range of temperatures, X-ray luminosities, and lifetimes against cooling. We predict the appearance of the X-ray background on a fine angular scale for two cases: formation of the clouds at z=2 and at z=4. Such an ensemble of protocluster hot gas clouds will create features in the X-ray background on a small angular scale, observable with X-ray telescopes now under construction. We present the results of a numerical experiment which predicts the appearance of X-ray images of these clouds with both 1' and 12'' resolution