CO(1–0) imaging reveals 10-kiloparsec molecular gas reservoirs around star-forming galaxies at high redshift

¹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
² Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands
³ SRON – Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ German Aerospace Center (DLR), Institute of Communications and Navigation, Wessling, Germany
⁶ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2 85748 Garching bei München, Germany
⁷ Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 4R2, Canada
⁸ Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Section 4, Roosevelt Road, Taipei 106216, Taiwan
⁹ International Centre for Radio Astronomy Research, University of Western Australia, 7 Fairway, Crawley, Perth, Australia
¹⁰ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
¹¹ Instituto de Astrofísica de Canarias, Vía Láctea, 39020 La Laguna (Tenerife), Spain
¹² Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro # 8701, Ex-Hda. San José de la Huerta, Morelia, Michoacán C.P. 58089, Mexico
¹³ Cosmic Dawn Center (DAWN), København, Denmark
¹⁴ National Radio Astronomy Observatory, Charlottesville, VA, USA
¹⁵ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
¹⁶ Max–Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

^⋆ Corresponding author: mrybak@strw.leidenuniv.nl

Received: 10 November 2024
Accepted: 22 June 2025

Abstract

Massive star-forming galaxies at high redshift require a supply of molecular gas from their gas reservoirs that is replenished by infall from the surrounding circumgalactic medium to sustain their immense star formation rates. Our knowledge of the extent and morphology of cold-gas reservoirs of early galaxies is still in its infancy, however. We present the results of stacking more than 80 hours of JVLA observations of CO(1–0) emission, which traces the cold molecular gas, in 19 z = 2.0 − 4.5 dusty star-forming galaxies from the AS2VLA survey. The visibility-plane stack reveals extended emission with a half-light radius of 3.8 ± 0.5 kpc, which is a factor of 2–3 more extended than the dust-obscured star formation and 1.4 ± 0.2× more extended than the stellar emission revealed by the JWST. Stacking the [C I](1–0) observations for 10 galaxies from our parent sample yielded a half-light radius ≤2.6 kpc, which is marginally smaller than CO(1–0). The CO(1–0) size is also comparable to that of the [C II] haloes detected around high-redshift star-forming galaxies. This suggests that these arise from molecular gas. Photo-dissociation region modelling indicates that the extended CO(1–0) emission arises from clumpy dense clouds and not from smooth diffuse gas. Our results show that the bulk (up to 80%) of the molecular gas in these galaxies resides outside the star-forming region with only a small part directly contributing to the star formation.