Reheating of the intergalactic medium at z>10

The reheating and reionization of the intergalactic medium by photoionizing radiation is considered. It is shown that in a low gas density universe a modest input, even at z≃100, can reionize the gas. The temperature of the intergalactic medium can be significantly affected even if the gas subsequently recombines. Over the range of redshifts considered, 0≼z≼100, the difference is small for gas recombining at z∼100, becoming much larger for small redshifts. The molecular hydrogen abundance after recombination is also calculated. Results are presented for an Ω = 1 universe with Ωg = 0.05, nHe/nH = 1/12 and h0 = 1/2.