Halo gas and galaxy disk kinematics of a volume-limited sample of Mg class II absorption-selected galaxies at z ∼ 0.1

Abstract

We have directly compared Mg II halo gas kinematics to the rotation velocities derived from emission/absorption lines of the associated host galaxies. Our 0.096 ≤ z ≤ 0.148 volume-limited sample comprises 13 ∼ L⊙ galaxies, with impact parameters of 12-90kpc from background quasar sight lines, associated with 11 Mg II absorption systems with Mg II equivalent widths 0.3 ≤ Wr (2796) ≤ 2.3For only 5/13 galaxies, the absorption resides to one side of the galaxy systemic velocity and trends to align with one side of the galaxy rotation curve. The remainder have absorption that spans both sides of the galaxy systemic velocity. These results differ from those at z 0.5, where 74% of the galaxies have absorption residing to one side of the galaxy systemic velocity. For all the z 0.1 systems, simple extended disk-like rotation models fail to reproduce the full Mg II velocity spread, implying that other dynamical processes contribute to the Mg II kinematics. In fact 55% of the galaxies are "counter-rotating" with respect to the bulk of the Mg II absorption. These Mg II host galaxies are isolated, have low star formation rates (SFRs) in their central regions (≲ 1 M ⊙yr-1), and SFRs per unit area well below those measured for galaxies with strong winds. The galaxy Na ID (stellar+ISM) and Mg Ib (stellar) absorption line ratios are consistent with a predominately stellar origin, implying kinematically quiescent interstellar media. These facts suggest that the kinematics of the Mg II absorption halos for our sample of galaxies are not influenced by galaxy-galaxy environmental effects, nor by winds intrinsic to the host galaxies. For these low-redshift galaxies, we favor a scenario in which infalling gas accretion provides a gas reservoir for low-to-moderate SFRs and disk/halo processes. © 2011. The American Astronomical Society. All rights reserved.