Low‐ and High‐Ionization Absorption Properties of MgiiAbsorption–selected Galaxies at Intermediate Redshifts. II. Taxonomy, Kinematics, and Galaxies

Abstract

We examine a sample of 45 Mg II absorption-selected systems over the redshift range 0.4-1.4 in order to better understand the range of physical conditions present in the interstellar and halo gas associated with intermediate redshift galaxies. Mg II and Fe II absorption profiles were observed at a resolution of 6 km s-1 with HIRES/Keck. Lyα and C IV data were measured in FOS spectra obtained from the Hubble Space Telescope archive (resolution 230 km s-1). We perform a multivariate analysis of Wr(Mg II), Wr(Fe II), Wr(C IV), and Wr(Lyα) (rest-frame equivalent widths) and the Mg II kinematic spread. There is a large range of high- to low-ionization properties and kinematics in intermediate-redshift absorbers, that we find can be organized into five categories: classic, C IV-deficient, single/weak, double, and damped Lyα/H I-rich. These categories arise, in part, because there is a strong connection between low-ionization kinematics and the location of an absorber on the Wr(C IV)-Wr(Mg II) plane. Using photoionization modeling, we infer that in most absorbers a significant fraction of the C IV arises in a phase separate from that giving rise to the Mg II. We show that many of the C IV profiles are resolved in the FOS spectra because of the velocity structure in the C IV gas. For 16 systems, the galaxy MK, MB, B-K, and impact parameters are measured. We compare the available absorption-line properties (taken from Paper I) to the galaxy properties but find no significant (greater than 3 σ) correlations, although several suggestive trends are apparent. We compare the locations of our intermediate redshift absorbers on the Wr(C IV)-Wr(Mg II) plane with those of lower and higher redshift data taken from the literature and find evidence for evolution that is connected with the Mg II kinematics seen in HIRES/Keck profiles of Mg II at z > 1.4. We discuss the potential of using the above categorizations of absorbers to understand the evolution in the underlying physical processes giving rise to the gas and governing its ionization phases and kinematics. We also discuss how the observed absorbing gas evolution has interesting parallels with scenarios of galaxy evolution in which mergers and the accretion of protogalactic clumps govern the gas physics and provide reservoirs for elevated star formation rates at high redshift. At intermediate and lower redshifts, the galaxy gaseous components and star formation rates may become interdependent and self-regulatory such that, at z ≤ 1, the kinematics and balance of high- and low-ionization gas may be related to the presence of star-forming regions in the host galaxy.