Competition between pressure and gravity confinement in Lyα forest observations

A break in the distribution function of Lyα clouds (at a typical redshift of 2.5) has been reported by Petitjean et al. (1993). This feature is what would be expected from a transition between pressure confinement and gravity confinement (as predicted in Charlton, Salpeter, & Hogan 1993). The column density at which the feature occurs has been used to determine the external confining pressure, ∼10 cm^-3 K, which could be due to a hot, intergalactic medium. For models that provide a good fit to the data, the contribution of the gas in clouds to Ω is small. The specific shape of the distribution function at the transition (predicted by models to have a nonmonotonic slope) can serve as a diagnostic of the distribution of dark matter around Lyα forest clouds, and the present data already eliminate certain models.