Possible Thermal Histories of Intergalactic Gas

Abstract

By using simple expressions to represent the time dependence of the heating of intergalactic gas, the thermal history of an initially cold gas is followed and the resulting radiation field is computed. If one requires that the gas must have been strongly heated and ionized at a redshift of about 2 to avoid strong Ly-a absorption in the spectra of QSO (here assumed to be cosmologically distant) then, at a final gas density of the order of 10 -5/cm1, inadmissibly large amounts of X-ray emission in the 5-10-A region can be avoided only for a very narrow range of heating parameters. Measures at 25-30 A with comparable sensitivity would be decisive. Models with a high degree of clumpiness in the gas can already be excluded. Koehler's reported value of an intergalactic hydrogen density of flH 10- cannot be reconciled with any of the models investigated, and it appears that one either has to drop the assumption of cosmological distances for the QSO or else consider a model in which the ionization is caused by a burst of star form ation in the early stages of galaxy formation. However, a specific model of this type, assuming stars of = 21000 K encounters the difficulty that the total light from the galaxies at this epoch should be far more intense than the observations allow.