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.

We analyze high-resolution spectra of a multicloud weak [defined as W r Mg ð iiÞ < 0:3 8] absorbing system along the line of sight to PG 1634+706. This system gives rise to a partial Lyman limit break and absorption in Mg ii, Si ii, C ii, Si iii, Si iv, C iv, and O vi. The lower ionization transitions arise in two kinematic subsystems with a separation of '150 km s À1 . Each subsystem is resolved into several narrow components having Doppler widths of 3-10 km s À1 . For both subsystems, the O vi absorption arises in a separate, higher ionization phase, in regions dominated by bulk motions in the range of 30-40 km s À1 . The two O vi absorption profiles are kinematically offset by '50 km s À1 with respect to each of the two lower ionization subsystems. In the stronger subsystem, the Si iii absorption is strong, with a distinctive, smooth profile shape, and may partially arise in shock-heated gas. Moreover, the kinematic substructure of Si iv traces that of the lower ionization Mg ii, but may be offset by '3 km s À1 . On the basis of photoionization models constrained by the partial Lyman limit break, we infer a low metallicity of $0.03 solar for the low-ionization gas in both subsystems. The broader O vi phases have a somewhat higher metallicity, and they are consistent with photoionization; the profiles are not broad enough to imply production of O vi through collisional ionization. Various models, including outer disks, dwarf galaxies, and superwinds, are discussed to account for the phase structure, metallicity, and kinematics of this absorption system. We favor an interpretation in which the two subsystems are produced by condensed clouds far out in the opposite extremes of a multilayer dwarf galaxy superwind.

We present the following results of photoionization modeling of six Mg iiselected absorption systems, at redshift 0.5 ≤ z ≤ 0.9, along lines of sight toward three quasars: PG 1241+176, PG 1248+401, and PG 1317+274. These are part of a larger sample of ∼ 20 Mg ii absorbers at intermediate redshift, that facilitates a survey of the properties of, and processes active in, the multiple phases of gas, both in and outside of galaxies. We present new high resolution ultraviolet spectra from HST/STIS as well as high resolution optical spectra obtained with HIRES/Keck. Together, these spectra allow simultaneous study of a variety of ionization states of different chemical elements, with resolution of the kinematics providing strong constraints on multiphase properties. The six systems presented here include several that are likely to arise from lines of sight through disk and halo structures of spiral galaxies. For these we find that the high ionization gas, traced by Civ is consistent with coronal structures that resemble that of the Milky Way, along with high velocity clouds at 100-200 km s −1 . Another system has relatively weak Civ, relative to the low ionization absorption, which leads to a discussion of the circumstances in which a corona might be weak or absent in a galaxy, i.e. an early-type morphology or low star formation rate. Finally, we present systems that are examples of single-cloud and multiple-cloud 1 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.

The z = 0.9902 system along the quasar PG 1634+706 line of sight is a strong Mg ii absorber (W r (2796) > 0.3 Å) with only weak Civ absorption (it is "Civdeficient"). To study this system, we used high-resolution spectra from both the Hubble Space Telescope (HST)/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, Cii, Si iii, Ciii, Si iv, and Civ. The HIRES spectrum, with a resolution of R = 45, 000, covers the Mg i, Mg ii and Feii transitions. Assuming a Haardt and Madau extragalactic background spectrum, we modeled the system with a combination of photoionization and collisional ionization. Based on a comparison of synthetic spectra to the data profiles, we infer the existence of the following four phases of gas: i) Seven Mg ii clouds have sizes of 1-1000 pc and densities of 0.002-0.1 cm −3 , with a gradual decrease in density from blue to red. The Mg ii phase gives rise to 1 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.

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