The kinematics of intermediate-redshift MgII absorbers

We present 23 quasar absorption-line systems selected by the Mg II lambda lambda 2796, 2803 doublet with W(r)(2796) > 0.3 Angstrom over the redshift range 0.4 less than or equal toz less than or equal to1.2. The kinematics and profile morphologies are studied at similar or equal to6 km s(-1) resolution to a 5 sigma equivalent width detection threshold of W(r)(2796) = 0.015 Angstrom. We are thus sensitive to very weak clouds isolated in velocity. The absorption profiles were segregated into kinematic subsystems, and the properties (velocities, velocity widths, column densities, etc.) were measured directly from the data and by Voigt profile modeling. Most absorbers are characterized by a dominant kinematic subsystem with W(r)(2796) > 0.2 Angstrom and velocity spreads ranging from 10 to 50 km s(-1) in proportion to the system equivalent width. Additional kinematic subsystems have velocities separations as large as 400 km s(-1) relative to the dominant subsystem. The equivalent widths and velocity spreads of these weaker subsystems are anticorrelated with their velocities, and their equivalent width distribution turns down from a power law below W(r)(2796) similar or equal to 0.08 Angstrom. These moderate - and high- velocity subsystems are inferred to have sub-Lyman limit H I, and therefore are probably not higher redshift analogs to Galactic high-velocity clouds (HVCs). Weak subsystems are asymmetrically distributed in velocity such that they are either all blueshifted or all redshifted with respect to the dominant subsystem. This implies, that although the kinematic morphologies vary greatly on a case-by-case basis, a given line of sight is apparently probing a well-defined spatial and kinematic structure. We investigate a simple kinematic model that relies on a rotating disk to explain the observed asymmetries. There are systematic differences, or trends, in both the subsystem-to-subsystem velocity clustering and in the overall kinematic morphologies with increasing equivalent width; we discuss how these may provide clues to the observed differential evolution in the equivalent width distribution of Mg II absorbers.