THE TEMPERATURE OF THE LYMAN-ALPHA CLOUDS AND THE ULTRAVIOLET IONIZING BACKGROUND AT HIGH REDSHIFTS

It is shown that it is possible to account for low temperatures in diffuse photoionized clouds using ionization arguments in a manner independent of the structure and confinement of the clouds. We emphasize that cooling by inverse Compton scattering of cosmic microwave background photons by electrons and the effect of a strong but progressive decrease of the UV ionizing background at wavelengths shorter than the He II edge can allow for temperatures as low as 20,000 K for gas in photoionization equilibrium. Using recent determination of H I column densities and temperatures from profile fitting of QSO Lyalpha absorption lines, it is shown that the total density of these high-redshift clouds lies in a narrow range around n approximately 10(-4) cm-3. The sizes are larger than 20 kpc. The observed temperature distribution is explained in terms of space fluctuations in the ionizing UV background at energies larger than the He II edge.