JWST MIRI/MRS observations of hot molecular gas in an AGN host galaxy at Cosmic Noon

Abstract

Active Galactic Nuclei (AGN) are believed to play a central role in quenching star formation by removing or destroying
molecular gas from host galaxies via radiation-pressure driven outflows and/or radio jets. Some studies of cold molecular gas
in galaxies at Cosmic Noon (z ∼ 2) show that AGN have less cold gas (<100 K) compared to mass-matched star-forming
galaxies. However, cold gas could also be shock-heated to warmer phases, detectable via H₂ transitions in the rest-frame near and mid-infrared spectra. The Medium Resolution Spectrograph (MRS) of the Mid-infrared Instrument (MIRI) aboard JWST
has opened a unique window to observe these emission lines in galaxies at Cosmic Noon. We present the first detection of hot
molecular gas in cid 346, an X-ray AGN at z ∼ 2.2, via the H2 ro-vibrational transition at 2.12 μm. We measure a hot molecular
gas mass of ∼ 8.0 × 10⁵ M⊙, which is ∼ 10⁵ − 10⁶ times lower than the cold molecular gas mass. cid 346 is located in an
environment with extended gas structures and satellite galaxies. This is supported by detection of hot and cold molecular gas
out to distances >10 kpc in MIRI/MRS and ALMA data, respectively and ancillary NIRCam imaging that reveals two satellite
galaxies at distances of ∼0.4 arcsec (3.3 kpc) and ∼0.9 arcsec (7.4 kpc) from the AGN. Our results tentatively indicate that while
the CO(3-2)-based cold gas phase dominates the molecular gas mass at Cosmic Noon, H₂ ro-vibrational transitions are effective
in tracing hot molecular gas locally in regions that may lack CO emission.

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