Compton heating of the intergalactic medium by the hard X-ray background

High-resolution hydrodynamics simulations of the Ly alpha forest in cold dark matter dominated cosmologies appear to predict line widths that are substantially narrower than those observed. Here we point out that Compton heating of the intergalactic gas by the hard X-ray background (XRB), an effect neglected in all previous investigations, may help to resolve this discrepancy. The rate of gain in thermal energy by Compton scattering will dominate over the energy input from hydrogen photoionization if the XRB energy density is similar to 0.2 x/[epsilon] times higher than the energy density of the UV background at a given epoch, where x is the hydrogen neutral fraction in units of 10(-6) and [epsilon] is the mean X-ray photon energy in units of m(e)c(2). The numerical integration of the time-dependent rate equations shows that the intergalactic medium approaches a temperature of about 1.5 x 10(4) K at z > 3 in popular models for the redshift evolution of the extragalactic background radiation. The importance of Compton heating can be tested experimentally by measuring the Ly alpha line width distribution as a function of redshift, thus the Ly alpha forest may provide a useful probe of the evolution of the XRB at high redshifts.