The Absorption Signature of Six Mg II-selected Systems over 0.5 <= z <= 0.9

We present the results of photoionization modeling of six Mg II-selected 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, which 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 the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST), as well as high-resolution optical spectra obtained with Keck HIRES. 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 C IV, is consistent with coronal structures that resemble those of the Milky Way, along with high-velocity clouds at 100-200 km s^-1. Another system has relatively weak C IV, 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 weak Mg II absorbers, which are as a class not likely to be within ~50 kpc of luminous galaxies. Based on the physical conditions inferred from models, we discuss the nature of these types of systems, which may trace outer regions of galaxies, dwarf galaxy satellites, or metal-rich regions of the intergalactic medium.

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.