A MUSE study of stellar and ionized-gas kinematics in central group galaxies

Abstract

Understanding the gas accretion and feedback processes in low-mass galaxy groups is crucial for bridging the gap between isolated ellipticals and cluster-dominant galaxies. We present new Multi Unit Spectroscopic Explorer (MUSE) observations of 17 nearby brightest group galaxies (BGGs) that reveal a complex and multiphase origin for their gas reservoirs. Warm ionized gas traced with Hα and NII emission lines is detected in 77% (13/17) of the sample, displaying diverse morphologies from rotating disks with and without tails as well as diffuse gas, rings, and filamentary structures. While most of our BGGs host regularly rotating stellar components, unlike brightest cluster galaxies (BCGs), their warm ionized gas exhibits a wide range of kinematics. Nearly half of the rotating gas disks are aligned with the stars, which is consistent with internal recycling. However, the presence of tails, diffuse gas, or rings in three of these systems suggests that these structures may retain signatures of past accretion or interactions despite their present-day kinematic alignment. The remaining disks and ring-like structures and most of the filamentary systems are misaligned, supporting an external origin through mergers, interactions, or cooling from their hot halos. Compact disks are consistent with centrally condensed low-angular-momentum gas and possibly linked to residual cooling. Most filamentary systems lie near the precipitation threshold (t_ ̊m cool /t_ ̊m eddy ∼ 1--2) and preferentially occupy the turbulence-dominated locus of the k-plot, supporting a condensation-driven origin consistent with chaotic cold accretion. All galaxies with detected X-ray halos, including those without warm ionized gas, exhibit low central entropy (rm <30, keV, 2 ) and short central cooling times (rm <3, Gyr), indicating that these thermodynamic conditions alone are not sufficient to predict the presence of multiphase gas and that additional processes, such as active galactic nuclei (AGN) or interaction-driven perturbations and turbulence-induced thermal instabilities, play a key role in regulating gas condensation. Together, these results indicate that no single mechanism dominates. Rather, BGGs are transitional systems where internal recycling, environmental accretion, and cooling-driven condensation coexist, linking isolated ellipticals and cluster-dominant galaxies.