The physical state of primordial intergalactic clouds

The physical state of low-density, primordial gas in intergalactic space at redshift z ⋍ 2.4 is examined. Convenient expressions for the rates of ionization, recombination, heating and cooling in hydrogen and helium are presented. When the rates of these 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 10−6−10−3 cm−3. In such a structure the concentration of observable neutral atoms has a much steeper gradient than does the total density. Hydrogen Lα absorption lines in distant QSOs can arise in the densest parts of a smooth distribution of intergalactic matter rather than in isolated clouds embedded in a rarified, uniform medium. Some properties of intergalactic clouds are discussed with reference to recent observations and equilibrium models.