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

TL;DR

This study investigates how O VI gas in the circumgalactic medium (CGM) kinematics relate to the rotation of star-forming galaxies at $z\approx0.2$, and how this compares to low-ion gas. It analyzes 18 galaxy–quasar pairs with sightlines near the galaxy major axes using HST/COS to trace O VI and other ions, complemented by ground-based spectroscopy to derive systemic redshifts and rotation curves. The main findings show that individual O VI velocity components do not directly track disk rotation, but O VI components matching low-ion centroids at small $d$ tend to corotate, suggesting a co-spatial, inner-CGM reservoir linked to the extended disk; gas at larger radii shows weaker rotational correlation due to higher turbulence. These results imply a multi-phase CGM where inner O VI is connected to disk angular momentum and accretion processes, while outer CGM remains more kinematically disturbed, informing models of gas flow and galaxy evolution.

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.

Figures (1)

• Figure 1: Ensemble of 18 sightlines. All selected galaxies have inclined disks with inclination angles $i \gtrsim 45$$^{\circ}$. Each sightline intersects the galaxy near the major axis with azimuthal angle $\alpha \leq 45$$^{\circ}$. The impact parameter $d$ of the quasar sightlines ranges between 20 and 284 kpc. The color of the markers represents the sightline impact parameter normalized by the galaxy virial radius. Most sightlines intersect the inner CGM within 100 kpc (i.e., within 0.5$r_\mathrm{vir}$).