COMPETITION BETWEEN PRESSURE AND GRAVITY CONFINEMENT IN LY-ALPHA FOREST OBSERVATIONS

A break in the distribution function of Lyalpha clouds (at a typical r edshift of 2.5) has been reported by Petit-jean et al. (1993). This fe ature is what would be expected from a transition between pressure con finement 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, approximately 10 c m-3 K, which could be due to a hot, intergalactic medium. For models t hat provide a good fit to the data, the contribution of the gas in clo uds to OMEGA 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 Ly alpha forest clouds, and the present data already eliminate certain mo dels.