A Quadruple-Phase Strong Mg II Absorber at z ~ 0.9902 toward PG 1634+706

Abstract

The z=0.9902 system along the quasar PG 1634+706 line of sight is a strong Mg II absorber [W_r(λ2796)>0.3 Å] with only weak C IV absorption (it is ``C IV-deficient''). To study this system, we used high-resolution spectra from both the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) and the Keck I telescope High Resolution Echelle Spectrometer (HIRES). The STIS spectrum has a resolution of R=30,000 and covers key transitions such as Si II, C II, Si III, C III, Si IV, and C IV. The HIRES spectrum, with a resolution of R=45,000, covers the Mg I, Mg II, and Fe II transitions. Assuming a Haardt & Madau extragalactic background spectrum, we modeled the system with a combination of photoionization and collisional ionization. Based on a comparison of synthetic spectra with the data profiles, we infer the existence of the following four phases of gas:1. Seven Mg II clouds have sizes of 1-1000 pc and densities of 0.002-0.1 cm<sup>-3</sup>, with a gradual decrease in density from blue to red. The Mg II phase gives rise to most of the C IV absorption and resembles the warm, ionized intercloud medium of the Milky Way.2. Instead of arising in the same phase as Mg II, Mg I is produced in separate, narrow components with b~0.75 km s<sup>-1</sup>. These small Mg I pockets (~100 AU) could represent a denser phase (~200 cm<sup>-3</sup>) of the interstellar medium (ISM), analogous to the small-scale structure observed in the Milky Way ISM.3. A ``broad phase'' with a Doppler parameter b~60 km s^-1 is required to consistently fit Lyα, Lyβ, and the higher order Lyman series lines. A low metallicity (logZ<~-2) for this phase could explain why the system is ``C IV-deficient'' and also why N V and O VI are not detected. This phase may be a galactic halo, or it could represent a diffuse medium in an early-type galaxy.4. The strong absorption in Si IV relative to C IV could be produced in an extra, collisionally ionized phase with a temperature of T~60,000 K. The collisional phase could exist in cooling layers that are shock heated by supernova-related processes.

Based in part on observations obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among Caltech, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Based in part on observations obtained with the NASA/ESA Hubble Space Telescope, which is operated by the STScI for the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.