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^-1) of Mg II and Fe II in combination with Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) spectra (R ∼ 230 km s^-1) to place constraints on the physical conditions (metallicities, ionization conditions, and multiphase 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 H I, Si II, C II, N II, Fe III, Si III, Si IV, N III, C III, C IV, S VI, N V, and O VI, with ionization potentials ranging from 13.6 to 138 eV. The multiple Mg II clouds exhibit complex kinematics and the C IV, N V, and O VI are exceptionally strong in absorption. We assumed that the Mg II clouds are photoionized by the extragalactic 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 Mg II clouds, including allowed ranges of cloud-to-cloud variations within a system. We find that the Mg II clouds, which have a typical size of approximately 100 pc, give rise to the Si IV, the majority of which arises in a single, very large (∼5 kpc), higher ionization cloud. However, the Mg II clouds cannot account for the strong C IV, N V, and O VI absorption. We conclude that the Mg II clouds are embedded in extended (10-20 kpc), highly ionized gas that gives rise to C IV, N V, and O VI; these are multiphase 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.