Gaseous galactic halos and quasi-stellar object absorption-line systems

We investigate a model of Lyman limit QSO absorption systems where they are produced in gaseous galactic halos with a two-phase structure: a hot phase at the halo virial temperature in approximate pressure equilibrium and a cold, photoionized phase in the form of clouds confined by the pressure of the hot medium and falling through it to the halo center, probably to accrete on a galactic disk. We show that the masses of these clouds must be over a relatively narrow range so that they are stable against both gravitational collapse and evaporation. These masses, near 10(6) M., also lead to a covering factor near unity. The hot phase is required to have a core radius such that its cooling time in the core is equal to the age of the halo, and the mass in the cold phase is determined by the rate at which the hot gas cools. We calculate the number of Lyman limit systems arising in halos of different masses in a CDM model and their impact parameters implied by these assumptions, and find them to be in reasonable agreement with observations. The evolution with redshift is correctly reproduced at z less than or similar to 2, while at higher redshift we predict fewer absorbers than observed. The observed low-ionization systems such as Mg II are well reproduced as arising from the photoionized phase, and are mostly in halos around massive galaxies, while C IV selected systems are predicted to be more commonly associated with lower mass galaxies and also massive galaxies at large impact parameters. Some C IV might also arise from conduction fronts at intermediate temperature at the boundaries of the clouds. The hot phase may give rise to detectable absorption lines in O VI, while the column densities predicted for other highly ionized species are low and difficult to observe.