A Quadruple‐Phase Strong MgiiAbsorber atz∼ 0.9902 toward PG 1634+706

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Abstract
The z = 0.9902 system along the quasar PG 1634+706 line of sight is a strong Mg II absorber [Wr(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:
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