The SAMI Galaxy Survey: Kinematics of stars and gas in brightest group galaxies; the role of group dynamics

TL;DR

This study investigates how the dynamical state of galaxy groups influences the stellar and gas kinematics of their brightest galaxies, revealing correlations with group relaxation indicators and suggesting different accretion histories.

Contribution

It provides the first evidence linking group dynamical state to BGG kinematic properties, using a large sample and multiple relaxation probes.

Findings

Misalignment between gas and stellar rotation axes is more common in unrelaxed groups.

Regular rotator kinematics are more prevalent in relaxed groups.

Group BGG offset is a stronger predictor of kinematic differences than luminosity gap.

Abstract

We study the stellar and gas kinematics of the brightest group galaxies (BGGs) in dynamically relaxed and unrelaxed galaxy groups for a sample of 154 galaxies in the SAMI galaxy survey. We characterize the dynamical state of the groups using the luminosity gap between the two most luminous galaxies and the BGG offset from the luminosity centroid of the group. We find that the misalignment between the rotation axis of gas and stellar components is more frequent in the BGGs in unrelaxed groups, although with quite low statistical significance. Meanwhile galaxies whose stellar dynamics would be classified as `regular rotators' based on their kinemetry are more common in relaxed groups. We confirm that this dependency on group dynamical state remains valid at fixed stellar mass and Sersic index. The observed trend could potentially originate from a differing BGG accretion history in virialised and evolving groups. Amongst the halo relaxation probes, the group BGG offset appears to play a stronger role than the luminosity gap on the stellar kinematic differences of the BGGs. However, both the group BGG offset and luminosity gap appear to roughly equally drive the misalignment between the gas and stellar component of the BGGs in one direction. This study offers the first evidence that the dynamical state of galaxy groups may influence the BGG's stellar and gas kinematics and calls for further studies using a larger sample with higher signal-to-noise.