Gaseous Galactic Halos and QSO Absorption Line Systems
  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. 
H. J. Mo (MPA, IAS) and J. Miralda-Escude (IAS)