Unveiling the Multi-phase CGM and ISM in MACS1931-26 with JWST and ALMA

Abstract

The circumgalactic medium (CGM) serves as the interface between galaxies and their cosmic environment, hosting the baryon cycle across a wide range of temperatures, densities, and energy scales. With its unprecedented sensitivity and spectral coverage, JWST is revolutionizing our view of this cycle by enabling direct detection of warm molecular hydrogen via mid-infrared rotational lines. We present a detailed analysis of the multi-phase molecular gas in the brightest cluster galaxy (BCG) of the cool-core cluster MACS1931-26 (z = 0.35), combining new **JWST/MIRI** and archival **ALMA** observations. This BCG hosts a powerful radio-loud AGN, elevated star formation, and one of the largest known H₂ reservoirs at this redshift. We trace cold molecular gas (10–100 K) using multiple CO and [CI] lines, finding highly excited gas in the ISM, similar to local LIRGs, while the CGM appears much less excited, pointing to distinct excitation sources. Our JWST data reveal warm H₂ (100–1000 K) spatially coincident with the CO-emitting gas and exhibiting comparable kinematics. Intriguingly, the CGM shows a higher H₂ excitation temperature than the ISM, suggesting the presence of more energetic heating mechanisms, including shocks and AGN-driven X-ray emission. This highlights the CGM as a key site of feedback-regulated gas transformation. Moreover, we will discuss our plans to use upcoming JWST Cycle 4 **NIRCam + MIRI** spectroscopy (2–28 μm) to perform comprehensive radiative transfer and shock modeling, aiming to constrain heating sources and baryon cycle in the CGM and ISM. This pilot study lays the groundwork for a broader framework to trace baryon cycling in cool-core BCGs, leveraging the synergy of JWST and cold gas tracers as a transformative tool for CGM studies.