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The z=0.9902 system along the quasar PG 1634+706 line of sight is a strongMgII absorber (W(2796)>0.3A) with only weak CIV absorption (it is``CIV-deficient''). To study this system, we used high-resolution spectra fromboth HST/STIS (R=30,000) and Keck/HIRES (R=45,000). These spectra cover keytransitions, such as MgI, MgII, FeII, SiII, CII, SiIII, CIII, SiIV, and CIV.Assuming a Haardt and Madau extragalactic background spectrum, we modeled thesystem with a combination of photoionization and collisional ionization. Basedon a comparison of synthetic spectra to the data profiles, we infer theexistence of the following four phases of gas: i) Seven MgII clouds have sizesof 1-1000pc and densities of 0.002-0.1/cm^3, with a gradual decrease in densityfrom blue to red. The MgII phase gives rise to most of the CIV absorption andresembles the warm, ionized inter-cloud medium of the Milky Way; ii) Instead ofarising in the same phase as MgII, MgI is produced in separate, narrowcomponents with b~0.75km/s. These small MgI pockets (~100AU) could represent adenser phase (~200/cm^3) of the interstellar medium (ISM), analogous to thesmall-scale structure observed in the Milky Way ISM; iii) A ``broad phase''with a Doppler parameter, b~60km/s, is required to consistently fit Ly-alpha,Ly-beta, and the higher-order Lyman-series lines. A low metallicity (log Z <=-2) for this phase could explain why the system is ``CIV-deficient'', and alsowhy NV and OVI are not detected. This phase may be a galactic halo or it couldrepresent a diffuse medium in an early-type galaxy; iv) The strong absorptionin SiIV relative to CIV could be produced in an extra, collisionally ionizedphase with a temperature of T~60,000K. The collisional phase could exist incooling layers that are shock-heated by supernovae-related processes.Comment: 25 pages, 4 figures; to appear in ApJ, April 20, 200

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