The Multiple Phases of Interstellar and Halo Gas in a Possible Group of   Galaxies at z~1
  We used HIRES/Keck profiles (R=6 km/s) of MgII and FeII in combination with FOS/HST spectra (R=230 km/s) to place constraints on the physical conditions (metallicities, ionization conditions, and multi-phase distribution) of absorbing gas in three galaxies at z=0.9254, 0.9276, and 0.9343 along the line of sight to PG 1206+459. The chemical and ionization species covered in the FOS/HST spectra are HI, SiII, CII, NII, FeIII, SiIII, SiIV, NIII, CIII, CIV, SVI, NV, and OVI, with ionization potentials ranging from 13.6 to 138 eV. The multiple MgII clouds exhibit complex kinematics and the CIV, NV and OVI are exceptionally strong in absorption. We assumed that the MgII clouds are photoionized by the extra-galactic background and determined the allowed ranges of their physical properties as constrained by the absorption strengths in the FOS spectra. A main result of this paper is that the low resolution spectra can provide meaningful constraints on the physical conditions of the MgII clouds, including allowed ranges of cloud to cloud variations within a system. We find that the MgII clouds, which have a typical size of ~100 pc, give rise to the SiIV, the majority of which arises in a single, very large (~5 kpc), higher ionization cloud. However, the MgII clouds cannot account for the strong CIV, NV, and OVI absorption. We conclude that the MgII clouds are embedded in extended (10-20 kpc), highly ionized gas that gives rise to CIV, NV, OVI; these are multi-phase absorption systems. The high ionization phases have near-solar metallicity and are consistent with Galactic-like coronae surrounding the individual galaxies, as opposed to a very extended common "halo" encompassing all three galaxies. 
Christopher W. Churchill and Jane C. Charlton (Penn State)