Compton Heating of the Intergalactic Medium by the Hard X-Ray Background

High-resolution hydrodynamics simulations of the Ly-alpha forest in cold darkmatter dominated cosmologies appear to predict line widths that aresubstantially narrower than those observed. Here we point out that Comptonheating of the intergalactic gas by the hard X-ray background (XRB), an effectneglected in all previous investigations, may help to resolve this discrepancy.The rate of gain in thermal energy by Compton scattering will dominate over theenergy input from hydrogen photoionization if the XRB energy density is0.2x/ times higher than the energy density of the UV background at agiven epoch, where x is the hydrogen neutral fraction in units of 1e-6 and is the mean X-ray photon energy in units of m_ec^2. The numericalintegration of the time-dependent rate equations shows that the intergalacticmedium approaches a temperature of about 1.5e4 K at z>3 in popular models forthe redshift evolution of the extragalactic background radiation. Theimportance of Compton heating can be tested experimentally by measuring theLy-alpha line-width distribution as a function of redshift, thus theLyman-alpha forest may provide a useful probe of the evolution of the XRB athigh redshifts.Comment: LaTeX, 10 pages, 2 figures, final version to be published in the Ap