Radiation Pressure in Lyman-α Forest Clouds

The effective recombination coefficient, αeff , is refined for optically thin cases. Radiation pressure in Lyman-α and HeII Lyman-α is calculated in terms of the number density and the mean free path. Pressure equilibrium between Lyman-α clouds and an adiabatically expanding inter-galactic medium is assumed, niTi = ncTc. Numerical models of isothermal and adiabatic expanding Lyman-α forest clouds are presented, including evolving UV sources (QSOs), with various turn-on times zon = 20, 10, and 5, and with q0 = 1 2 in a matterdominated Friedmann-Robertson-Walker Universe. These models lead to the conclusion that the radiation pressure and QSO turn-on time are significant in determining the range of physical size, D, and neutral hydrogen column density, N(HI), permitted for stable Lyman-α forest clouds. This manuscript was written in 1989 and never submitted for publication.