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

TL;DR

This study uses JWST MIRI/MRS observations to detect and analyze hot molecular gas in a high-redshift AGN host galaxy, revealing its properties and spatial distribution, and highlighting the importance of infrared spectroscopy in understanding galaxy evolution.

Contribution

First detection of hot molecular gas in a $z\sim2.2$ AGN using JWST MIRI/MRS, showing hot gas is much less abundant than cold gas and can trace regions lacking CO emission.

Findings

Hot molecular gas mass is about 10^5-10^6 times lower than cold gas.

Hot and cold gas extend over >10 kpc, indicating large-scale gas structures.

Infrared H2 transitions effectively trace hot molecular gas in AGN host galaxies.

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 ($z\sim2$) show that AGN have less cold gas ($<$100 K) compared to mass-matched star-forming galaxies. However, cold gas could also be shock-heated to warmer phases, detectable via H$_{2}$ 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 $z\sim2.2$, via the H$_{2}$ ro-vibrational transition at 2.12 $\mu$m. We measure a hot molecular gas mass of $\sim 8.0 \times 10^{5}$ M$_{\odot}$, which is $\sim 10^{5}-10^{6}$ 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 $\sim$0.4 arcsec (3.3 kpc) and $\sim$0.9 arcsec (7.4 kpc) from the AGN. Our results tentatively indicate that while the CO(3-2)-based cold gas phase dominates the molecular gas mass at Cosmic Noon, H$_{2}$ ro-vibrational transitions are effective in tracing hot molecular gas locally in regions that may lack CO emission.