2026ApJ...996...86G

We perform one of the first spatially resolved studies of warm (>100 K) and cold (10-100 K) molecular gas in the circumgalactic medium (CGM), focusing on the brightest cluster galaxy (BCG) of a cool-core galaxy cluster, MACS 1931-26, at z = 0.35. This galaxy has a massive H₂ reservoir and a radio-loud active galactic nucleus and is undergoing a starburst event. We present new JWST observations of this system, revealing warm H₂ gas that is cospatial with the cold molecular gas traced by CO, extending over 30 kpc around the BCG in a tail-like structure reaching into the CGM of this galaxy. Analysis of the mid-infrared pure H₂ rotational lines H₂ S(1), H₂ S(5), and H₂ S(9) indicate warm gas temperatures of 515.6 ± 0.8 and 535.2 ± 1.9 K in the BCG and tail regions, respectively. We compare the cold gas traced by the CO(3-2) observed with the Atacama Large Millimeter/submillimeter Array to the warm gas traced by JWST. The warm-to-cold molecular gas fraction is comparable in the BCG (1.4% ± 0.2%) and the CGM tail (1.9% ± 0.3%). Our analysis suggests that the dissipation of the kinetic energy of the H₂-emitting gas is sufficient to lead to the formation of the CO gas. This observation provides new insights into the molecular gas distribution and its potential role in the interplay between the central galaxy and its circumgalactic environment.