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A model of Lyman limit QSO absorption systems is investigated where they areproduced in gaseous galactic halos with a two-phase structure: a hot phase atthe halo virial temperature in approximate pressure equilibrium, and a cold,photoionized phase in the form of clouds confined by the pressure of the hotmedium, and falling through it to the halo center. We show that the masses ofthese clouds must be over a relatively narrow range so that they are stableagainst gravitational collapse and evaporation. These masses, near $10^6\msun$, also lead to a covering factor near unity. The hot phase is required tohave a core radius such that its cooling time in the core is equal to the ageof the halo, and the mass in the cold phase is determined by the rate at whichthe hot gas cools. We calculate the number of Lyman limit systems arising inhalos of different masses in a CDM model and their impact parameters implied bythese assumptions, and find them to be in reasonable agreement withobservations. The observed low ionization systems such as MgII are wellreproduced as arising from the photoionized phase, and are mostly in halosaround massive galaxies, while CIV selected systems are predicted to be morecommonly associated with lower mass galaxies and larger impact parameters. Thehot phase may give rise to detectable absorption lines in OVI, while the columndensities predicted for other highly ionized species are low and difficult toobserve.Comment: 44 pages, Latex(aaspp4.sty), 16 encap. ps figs. ApJ submitte

Gaseous Galactic Halos and Quasi-stellar Object Absorption-Line Systems