The Highly Ionized Circumgalactic Medium is Kinematically Uniform Around Galaxies

TL;DR

This study reveals that the highly ionized circumgalactic medium, traced by OVI absorption, exhibits uniform kinematic properties around galaxies regardless of galaxy orientation or star formation activity, suggesting complex ionization conditions.

Contribution

It introduces a pixel-velocity two-point correlation function analysis to characterize OVI absorber kinematics and compares it with MgII, highlighting differences in ionization and gas dynamics.

Findings

OVI absorption velocity spreads are more extended than MgII.

OVI kinematics are similar across galaxy properties, unlike MgII.

High ionization gas kinematics are not strongly affected by star formation.

Abstract

The circumgalactic medium (CGM) traced by OVI $\lambda\lambda 1031, 1037$ doublet absorption has been found to concentrate along the projected major and minor axes of the host galaxies. This suggests that OVI traces accreting and outflowing gas, respectively, which are key components of the baryon cycle of galaxies. We investigate this further by examining the kinematics of 29 OVI absorbers associated with galaxies at $0.13 < z_{\rm gal} < 0.66$ as a function of galaxy color, inclination, and azimuthal angle. Each galaxy was imaged with HST and the absorption was detected in COS/HST spectra of nearby ($D<200$ kpc) background quasars. We use the pixel-velocity two-point correlation function to characterize the velocity spread of the absorbers, which is a method used previously for a sample of MgII absorber--galaxy pairs. The absorption velocity spread for OVI is more extended than MgII, which suggests that the two ions trace differing components of the CGM. Also contrary to MgII, the OVI absorption velocity spreads are similar regardless of galaxy color, inclination, and azimuthal angle. This indicates that the kinematics of the high ionization gas is not strongly influenced by the present star formation activity in the galaxy. The kinematic homogeneity of OVI absorption and its tendency to be observed mainly along the projected galaxy major and minor axes is likely due to varying ionization conditions and gas densities about the galaxy. Gas in intermediate azimuthal angles may be ionized out of the OVI phase, possibly resulting in an azimuthal angle dependence on the distribution of gas in higher ionization states.