The physical properties of primordial intergalactic gas ? re-examination of self-gravitating models of lyman-alpha forest

The physical state of optically thin, primordial gas in intergalactic space at redshift about Z=2.5 is re-examined, assuming the gas is in ionzation and thermal equilibrium by the integrated UV flux from QSOs. For densities in the range 10−5–10−2 cm−3, the structure of the gas can be well described by an isothermal polytrope of infinite index. The Lα absorbing clouds observed in the spectra of distant QSOs may arise in such self-gravitating intergalactic clouds. Some properties of the clouds are discussed with reference to recent observations and equilibrium models.