High‐Resolution STIS/Hubble Space Telescopeand HIRES/Keck Spectra of Three Weak MgiiAbsorbers toward PG 1634+706

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Abstract
High-resolution optical (HIRES/Keck) and UV (STIS/Hubble Space Telescope) spectra, covering a large range of chemical transitions, are analyzed for three single-cloud weak Mg II absorption systems along the line of sight toward the quasar PG 1634+706. Weak Mg II absorption lines in quasar spectra trace metal-enriched environments that are rarely closely associated with the most luminous galaxies (>0.05L*). The two weak Mg II systems at z = 0.81 and 0.90 are constrained to have at least solar metallicity, while the metallicity of the z = 0.65 system is not as well constrained, but is consistent with more than 1/10 solar. These weak Mg II clouds are likely to be local pockets of high metallicity in a lower metallicity environment. All three systems have two phases of gas, a higher density region that produces narrower absorption lines for low-ionization transitions, such as Mg II, and a lower density region that produces broader absorption lines for high-ionization transitions, such as C IV. The C IV profile for one system (at z = 0.81) can be fitted with a single broad component (b ~ 10 km s-1), but those for the other two systems require one or two additional offset high-ionization clouds. Two possible physical pictures for the phase structure are discussed: one with a low-ionization, denser phase embedded in a lower density surrounding medium and the other with the denser clumps surrounding more highly ionized gas.
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