COS-EDGES: Co-rotation and Kinematic Stratification of the Multi-Phase CGM Around Edge-On Galaxies

TL;DR

This study investigates the kinematic connection between the interstellar medium and multi-phase circumgalactic medium in edge-on galaxies, revealing a stratified structure with co-rotation in the inner halo and diverse motions at larger radii.

Contribution

It provides the first detailed analysis of CGM kinematics around edge-on galaxies, demonstrating radial dependence and ionisation state differences in co-rotation and velocity dispersion.

Findings

Over 80% of low-ionisation absorption co-rotates within 0.2 R_vir.

High-ionisation gas shows weaker co-rotation and lower velocities beyond 0.2 R_vir.

Velocity widths are larger in the inner halo for low-ionisation gas.

Abstract

We present the first results from the COS-EDGES survey, targeting the kinematic connection between the ISM and multi-phase circumgalactic medium (CGM) in nine isolated, edge-on galaxies at z~0.2, each probed along its major axis by a background quasar at impact parameters of 13-38kpc. Using VLT/UVES and HST/COS quasar spectra, we analyse MgI, MgII, HI, CII, CIII, and OVI absorption relative to galaxy rotation curves from Keck/LRIS and Magellan/MagE spectra. We find that at lower $D/R_{vir}$ ($D/R_{vir}\leq 0.2$), over 80% of absorption in all ions lies on the side of systemic velocity matching disk rotation, and the optical-depth-weighted median velocity ($v_{abs}$) is consistent with the peak rotation speed. At higher $D/R_{vir}$ ($D/R_{vir} > 0.2$), the kinematics diverge by ionisation state: For low ionisation gas, the amount of co-rotating absorption remains >80%, yet $v_{abs}$ drops to 60% of the galaxy rotation speed. For high ionisation gas (OVI), only 60% of the absorption is consistent with co-rotation and $v_{abs}$ drops to 20% of the rotation speed. Furthermore, the velocity widths, corresponding to 50% of the total optical depth ($\Delta v_{50}$) for low ionisation gas is 1.8 times larger in the inner halo than at larger radii, while for CIII and OVI $\Delta v_{50}$ remains unchanged with distance. These results suggest a radially dependent CGM kinematic structure: the inner halo hosts cool, dynamically broad gas tightly coupled to disk rotation, whereas beyond 0.2$R_{vir}$, particularly traced by OVI and HI, the CGM shows weaker rotational alignment and lower velocity dispersion. Therefore, low-ionisation gas likely traces extended co-rotating gas, inflows and/or recycled accretion, while high-ionisation gas reflects a mixture of co-rotating, lagging, discrete collisionally ionised structures, indicating a kinematic stratification of the multi-phase CGM. [Abridged]