The physical state of primordial intergalactic clouds

The equations of ionization and thermal equilibrium have been solved for a low-density photoionized gas of hydrogen and helium in intergalactic space at a red shift of approximately 2.4. When the rates of ionization, recombination, heating, and cooling processes are in steady state, the structure of the gas approaches that of a polytrope of index N = -3.5 for densities in the range 0.000001-0.001 per cu cm. In such a structure the concentration of observable neutral atoms has a much steeper gradient than does the total density. The properties of such configurations are compared with observations of QSO absorption lines attributed to isolated intergalactic clouds; and it is found that hydrogen Lyman-alpha lines in distant QSOs can arise in the densest part of a smooth distribution of intergalactic matter rather than in isolated clouds embedded in a rarefied uniform medium. Properties of intergalactic clouds are examined in relation to recent observations and equilibrium models.