Halo Gas and Galaxy Disk Kinematics of a Volume-Limited Sample of MgII Absorption-Selected Galaxies at z~0.1

TL;DR

This study compares MgII halo gas kinematics with galaxy rotation in a low-redshift sample, revealing complex dynamics not explained by simple disk rotation models, and suggesting infalling gas as a key process.

Contribution

It provides the first detailed comparison of MgII halo gas kinematics with galaxy rotation at z~0.1, highlighting the role of non-rotational dynamics and infall processes.

Findings

Many MgII absorptions do not align with galaxy rotation.

Over half of the galaxies show counter-rotation relative to MgII absorption.

Simple disk rotation models cannot explain the MgII velocity spread.

Abstract

We have directly compared MgII 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 12-90 kpc from background quasars sight-lines, associated with 11 MgII absorption systems with MgII equivalent widths 0.3< W_r(2796)<2.3A. For 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 MgII velocity spread, implying other dynamical processes contribute to the MgII kinematics. In fact 55% of the galaxies are "counter-rotating" with respect to the bulk of the MgII absorption. These MgII host-galaxies are isolated, have low star formation rates (SFRs) in their central regions (<1 Msun/yr), and SFRs per unit area well below those measured for galaxies with strong winds. The galaxy NaID (stellar+ISM) and MgIb (stellar) absorption line ratios are consistent with a predominately stellar origin, implying kinematically quiescent interstellar media. These facts suggest that the kinematics of the MgII 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 star formation rates and disk/halo processes.