MAGIICAT VI. The MgII Intragroup Medium is Kinematically Complex

TL;DR

This study compares MgII absorption in galaxy group environments to isolated galaxies, revealing that the intragroup medium is kinematically complex and influenced by multiple galaxies, with implications for understanding galaxy interactions and gas transfer.

Contribution

It provides the first detailed comparison of MgII absorption properties between galaxy groups and isolated galaxies, highlighting the complex kinematics and potential origins of intragroup gas.

Findings

Group absorbers have larger median equivalent widths and covering fractions than isolated ones.

Group environment kinematics are similar to isolated environments but show more high-velocity dispersion features.

Gas in groups is likely coupled to the entire group, not just individual galaxies.

Abstract

By comparing MgII absorption in the circumgalactic medium (CGM) of group environments to isolated galaxies, we investigated the impact of environment on the CGM. A MgII absorber is associated with a group if there are two or more galaxies at the absorption redshift within a projected distance of D=200 kpc from a background quasar and a line-of-sight velocity separation of 500 km/s. We compiled a sample of 29 group environments consisting of 74 galaxies (2-5 galaxies per group) at $0.113<z_{\rm gal}<0.888$. The group absorber median equivalent width ($\langle W_r(2796)\rangle=0.65\pm0.13$ {\AA}) and covering fraction ($f_c=0.89_{-0.09}^{+0.05}$) are larger than isolated absorbers ($1.27\sigma$ and $2.2\sigma$, respectively) but median column densities are statistically consistent. A pixel-velocity two-point correlation function analysis shows that group environment kinematics are statistically comparable to isolated environments ($0.8\sigma$), but with more power for high velocity dispersions similar to outflow kinematics. Group absorbers display more optical depth at larger velocities. A superposition model in which multiple galaxies contribute to the observed gas matches larger equivalent width group absorbers, but overpredicts the kinematics significantly due to large velocity separations between member galaxies. Finally, galaxy--galaxy groups (similar member galaxy luminosities) may have larger absorber median equivalent widths ($1.7\sigma$) and velocity dispersions ($2.5\sigma$) than galaxy--dwarf groups (disparate luminosities). We suggest the observed gas is coupled to the group rather than individual galaxies, forming an intragroup medium. Gas may be deposited into this medium by multiple galaxies via outflowing winds undergoing an intergalactic transfer between member galaxies or from tidal stripping of interacting members.