Direct detection of cool molecular gas in a star-forming galaxy at z=7.31

Cescon, K, Hodge, JA, Boogaard, LA, Algera, HSB, Rowland, LE, Riechers, DA, Smit, R ORCID: 0000-0001-8034-7802, De Looze, I, Bouwens, R, van der Werf, P, Aravena, M, da Cunha, E, Dayal, P, Ferrara, A, Fisher, R, Inami, H, Mancera Piña, PE, Oesch, PA, Pallottini, A, Rybak, M et al (2026) Direct detection of cool molecular gas in a star-forming galaxy at z=7.31. Monthly Notices of the Royal Astronomical Society, 549 (3). ISSN 0035-8711

Preview

Text Direct detection of cool molecular gas in a star-forming galaxy at z=7.31.pdf - Published Version Available under License Creative Commons Attribution. Download (4MB) | Preview

Abstract

We investigate the molecular gas content and interstellar medium (ISM) conditions of REBELS-25, a massive, starforming galaxy at z = 7.31. Deep VLA (Very Large Array) Q-band and Atacama Large Millimeter/submillimeter Array (ALMA) Band 3 observations reveal CO(3−2) and CO(7−6) emission (both at ∼ 3.5σ), and provide an upper
limit on [C i](2−1). From the CMB (cosmic microwave background)-corrected CO(3−2) flux – representing the highest redshift detection of a low-J CO transition to date – we derive a molecular gas mass of Mmol = (1.0 ± 0.4) ×1011 (αCO/(3 M(K km s−1 pc−2 )−1 )) M, directly confirming the presence of a very massive gas reservoir only 700 Myr afterthe big bang. Thisimplies an extreme gasfraction of fgas 0.95, a gas-to-dustratio of δGDR 6 × 102, and a depletion time-scale of τdep 1.2 Gyr, broadly consistent with extrapolated scaling relations for main-sequence galaxies at lower redshift. Using the radiative transfer code tuner, we self-consistently model CO and dust continuum emission in the context of the significant CMB background, constraining ISM properties and recovering Mmol = (1.8+1.0−0.9) × 1011 M, independent of assumptions aboutr31 and αCO. We further discussthe use of alternative molecular gastracers at early epochs.
Combining CO and [C ii] measurements, we infer an empirical [C ii]-to-H2 conversion factor of α[C ii] = (60 ± 25) M/L, suggesting [C ii] remains a viable molecular gas tracer in the epoch of reionization. These results demonstrate the detectability of low-J CO emission even at z > 7, paving the way for next-generation facilities, and provide critical insights into the rapid mass assembly of galaxies during the first billion years of cosmic history.

Item Type:	Article
Uncontrolled Keywords:	galaxies: evolution; galaxies: high-redshift; galaxies: ISM; galaxies: star formation; radio lines: galaxies; submillimetre: galaxies; 5101 Astronomical Sciences; 51 Physical Sciences; 7 Affordable and Clean Energy; 5101 Astronomical Sciences; 51 Physical Sciences; 7 Affordable and Clean Energy; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press
Date of acceptance:	13 May 2026
Date of first compliant Open Access:	15 September 2026
Date Deposited:	15 Sep 2026 14:53
Last Modified:	15 Sep 2026 14:53
DOI or ID number:	10.1093/mnras/stag924
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29438

View Item