Physical properties of the intergalactic medium and the Lyman-alpha absorbing clouds

Following the interpretation of Sargent et al. (1980) that the Lyman-alpha absorbing systems seen in quasars are due to intergalactic clouds confined by the pressure of a general intergalactic medium, the physical properties of that medium and of the clouds are determined on the basis of a variety of theoretical and observational constraints, with best estimates of the present intergalactic pressure and temperature being P/k = 10 exp -2 + or - 0.2/cu cm and T = 10 exp 4.5 + or - 0.5 K. It is also shown that a model of the intergalactic medium based on UV heating fails and that some explosive events must have occurred which shock-heated the medium during the epoch z = 3-7. Three specific, if difficult, observations are proposed to check the interpretation developed: a search for the He(+) Gunn-Peterson line at 304 A, a better determination of the Doppler width of the normal Lyman-alpha lines, and a search for low equivalent width Lyman-alpha lines.