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

D. Kakkad, V. Mainieri, Takumi S. Tanaka, John D. Silverman, D. Law, Rogemar A. Riffel, C. Circosta, E. Bertola, M. Bianchin, M. Bischetti, G. Calistro Rivera, S. Carniani, C. Cicone, G. Cresci, T. Costa, C. M. Harrison, I. Lamperti, B. Kalita, Anton M. Koekemoer, A. Marconi, M. Perna, E. Piconcelli, A. Puglisi, Gabriele S. Ilha, G. Tozzi, G. Vietri, C. Vignali, S. Ward, G. Zamorani, L. Zappacosta

First listed 2025-07-07 | Last updated 2025-07-07

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 $μ$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.

Short digest

Using JWST/MIRI MRS, the authors detect ro‑vibrational H2 in the z≈2.2 X‑ray AGN cid_346, measuring 1‑0 S(1) (and S(0)) and inferring a hot H2 mass of ≈8×10^5 M⊙. They map hot molecular emission and CO(3‑2) to >10 kpc, and NIRCam imaging reveals two nearby satellites at ≈3.3 and 7.4 kpc, pointing to an interacting environment. The hot phase is 10^5–10^6× less massive than the cold reservoir, but the authors argue H2 ro‑vibrational lines effectively trace locally heated/shocked gas where CO may be faint. Ionized gas shows [O III] extension and a blue wing toward the SE, consistent with outflow activity shaping the multiphase medium.

Key figures to inspect

• Figure 1: Use the four context panels to place cid_346 on the star‑forming main sequence, above the local MBH–M* relation, and within the Fabian (2008) NH–λEdd outflowing regime—together arguing it can drive winds at Cosmic Noon.
• Figure 2: Compare the [O III] flux map and blue‑wing spectrum with the ALMA CO(3‑2) map/spectrum to judge the geometry and kinematics of the ionized outflow (SE extension) relative to the cold molecular gas.
• Figure 3: Inspect the 1‑0 S(1) and 1‑0 S(0) line fits (centroids, widths, and 95% flux windows) that underpin the hot‑H2 detection and mass estimate; check S/N and any velocity offset from systemic.
• Figure 4: Read the 1‑0 S(1)/1‑0 S(0) ratio versus Lbol placement of cid_346 against low‑z Seyferts to assess the likely excitation (thermal/shock vs fluorescent) in this source.