Kinematics of Circumgalactic O VI Gas and Disk Rotation of $z\approx0.2$ Star-forming Galaxies

TL;DR

This study investigates the kinematics of O VI gas in the circumgalactic medium of star-forming galaxies at z≈0.2, revealing that O VI often co-rotates with the galaxy disk and is linked to the low-ion gas near the extended disk.

Contribution

It provides new insights into the relationship between O VI absorption and galaxy disk rotation, especially regarding the kinematic connection at different impact parameters.

Findings

O VI gas rarely counter-rotates relative to the disk.

O VI components matching low ions tend to be near the galaxy and co-rotate.

Large-radius O VI gas shows weaker correlation with disk rotation due to turbulence.

Abstract

Quasar sightline observations reveal that low-ionization-state gas corotates with the galaxy disk and often at sub-centrifugal velocities, suggesting that the gas is spiraling towards the galaxy disk. However, while observations ubiquitously detect O VI absorption around low-redshift, $\sim L^*$ star-forming galaxies, the relationship between O VI and the galaxy disk, especially the kinematics, is not well-established. This work focuses on the O VI kinematics and its comparison with that of the low ions and galactic disk rotation. We present observations of 18 pairs of quasars and $z\approx0.2$ star-forming galaxies. All quasar sightlines intersect the circumgalactic medium (CGM) within 45$^\circ$ from the galaxy major axes. We show that while individual O VI velocity components do not correlate with disk rotation, the bulk of O VI gas in individual sightlines rarely counter-rotates. We then match O VI velocity components with those of low ions by minimizing the difference of their velocity centroids. The O VI velocity components with successful low-ion matches are typically found at small sightline impact parameters and are more likely to corotate with the disk. We suggest that the low-ion-matched O VI velocity components trace the gas co-spatial with the low ions near the extended disk plane in the inner CGM, whereas those without low-ion matches represent the gas at large 3D radii. While the gas at large radii is theoretically expected to kinematically correlate with the disk angular momentum, this correlation is expected to be weaker due to the higher turbulent to mean rotation velocity ration at large radii, consistent with our results.