MULTIPHASE GAS IN QUASAR ABSORPTION-LINE SYSTEMS

In the standard model for H I Lyman-limit (LL) quasar absorption-line systems, the absorbing matter is galactic disk and halo gas, heated an d photoionized by the metagalactic radiation field produced by active galaxies. In recent Hubble Space Telescope observations (Reimers et al . 1992; Vogel and Reimers 1993; Reimers and Vogel 1993) of LL systems along the line of sight to the quasar HS 1700+6416, surprisingly high He I/H I ratios and a wide distribution of column densities of C, N, a nd O ions are deduced from extreme ultraviolet absorption lines. We sh ow that these observations are incompatible with photoionization equil ibrium by a single metagalactic ionizing background. We argue that the se quasar absorption systems possess a multiphase interstellar medium similar to that of our Galaxy, in which extended hot, collisionally io nized gas is responsible for some or all of the high ionization stages of heavy elements. From the He/H ratios we obtain -4.0 less than or e qual to log U less than or equal to -3.0, while the CNO ions are consi stent with hot gas in collisional ionization equilibrium at log T = 5. 3 and [O/H] = -1.6. The supernova rate necessary to produce these heav y elements and maintain the hot-gas energy budget of similar to 10(41. 5) ergs s(-1) is similar to 10(-2) yr(-1), similar to that which maint ains the ''three-phase'' interstellar medium in our own Galaxy. As a c onsequence of the change in interpretation from photoionized gas to a multiphase medium, the derived heavy-element abundances (e.g., O/C) of these systems are open to question owing to substantial ionization co rrections for unseen C v in the hot phase. The metal-line ratios may a lso lead to erroneous diagnostics of the shape of the metagalactic ion izing spectrum and the ionizing parameter of the absorbers.