GASEOUS GALACTIC HALOS AND QUASI-STELLAR OBJECT ABSORPTION-LINE SYSTEMS

We investigate a model of Lyman limit QSO absorption systems where the y are produced in gaseous galactic halos with a two-phase structure: a hot phase at the halo virial temperature in approximate pressure equi librium 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 c enter, 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 a re stable against both gravitational collapse and evaporation. These m asses, near 10(6) M., also lead to a covering factor near unity. The h ot 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 calcula te the number of Lyman limit systems arising in halos of different mas ses in a CDM model and their impact parameters implied by these assump tions, 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 h alos around massive galaxies, while C IV selected systems are predicte d to be more commonly associated with lower mass galaxies and also mas sive galaxies at large impact parameters. Some C IV might also arise f rom conduction fronts at intermediate temperature at the boundaries of the clouds. The hot phase may give rise to detectable absorption line s in O VI, while the column densities predicted for other highly ioniz ed species are low and difficult to observe.