  A model of Lyman limit QSO absorption systems is investigated 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. We show that the masses of
these clouds must be over a relatively narrow range so that they are stable
against gravitational collapse and evaporation. These masses, near $10^6
\msun$, 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 observed low ionization systems such as MgII are well
reproduced as arising from the photoionized phase, and are mostly in halos
around massive galaxies, while CIV selected systems are predicted to be more
commonly associated with lower mass galaxies and larger impact parameters. The
hot phase may give rise to detectable absorption lines in OVI, while the column
densities predicted for other highly ionized species are low and difficult to
observe.
