ar X iv : a st ro - p h / 99 02 08 0 v 1 4 F eb 1 99 9 Compton Heating of the Intergalactic Medium by the Hard X – ray Background

High-resolution hydrodynamics simulations of the Lyα 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 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 ∼ 0.2x/〈ǫ〉 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 and 〈ǫ〉 is the mean X–ray photon energy in units of mec. The numerical integration of the time–dependent rate equations shows that the intergalactic medium approaches a temperature of about 2 × 10 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 Lyα line–width distribution and the equation of state of the intergalactic medium as a function of redshift. The Lyα forest may thus provide an important probe of the evolution of the XRB at high redshifts. Subject headings: cosmology: theory – diffuse radiation – intergalactic medium – quasars: absorption lines – X–rays: general