Clumpiness of galaxies revealed in the near-infrared with COSMOS-Web

Substructures at 1 < z < 4 and their link to stellar mass and star formation

¹ Aix Marseille Univ, CNRS, CNES, LAM Marseille, France
² Kavli Institute for Astronomy and Astrophysics, Peking University Beijing 100871, People’s Republic of China
³ Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa Chiba 277-8583, Japan
⁴ Cosmic Dawn Center (DAWN), Denmark
⁵ Niels Bohr Institute, University of Copenhagen Jagtvej 128 2200 Copenhagen, Denmark
⁶ Institut d’Astrophsyique de Paris, UMR 7095, CNRS, UPMC Univ. Paris VI 98 bis boulevard Arago Paris, France
⁷ Department of Physics and Astronomy, University of California, Riverside 900 University Avenue Riverside CA 92521, USA
⁸ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology 84 Lomb Memorial Drive Rochester NY 14623, USA
⁹ Caltech/IPAC, MS 314-6 1200 E. California Blvd. Pasadena CA 91125, USA
¹⁰ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS 75014 Paris, France
¹¹ Department of Astronomy, School of Science, The University of Tokyo 7-3-1 Hongo Bunkyo Tokyo 113-0033, Japan
¹² Institute of Physics, GalSpec, Ecole Polytechnique Federale de Lausanne, Observatoire de Sauverny Chemin Pegasi 51 1290 Versoix, Switzerland
¹³ INAF, Astronomical Observatory of Trieste Via Tiepolo 11 34131 Trieste, Italy
¹⁴ Instituto de Astrofísica de Canarias C/ Vía Láctea s/n 38205 La Laguna Tenerife, Spain
¹⁵ Departamento de Astrofísica, Universidad de La Laguna 38200 La Laguna Tenerife, Spain
¹⁶ Observatoire de Paris, LERMA, PSL University 61 avenue de l’Observatoire F-75014 Paris, France
¹⁷ Université Paris-Cité 5 Rue Thomas Mann 75014 Paris, France
¹⁸ Department of Astronomy, The University of Washington Seattle WA 98195, USA
¹⁹ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA
²⁰ Department of Computer Science, Aalto University P.O. Box 15400 FI-00076 Espoo, Finland
²¹ Department of Physics, University of P.O. Box 64 FI-00014 Helsinki, Finland
²² Department of Physics and Astronomy, UCLA, PAB 430 Portola Plaza Box 951547 Los Angeles CA 90095-1547, USA
²³ Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive Pasadena CA 91001, USA
²⁴ Department of Physics, University of California, Santa Barbara Santa Barbara CA 93106, USA
²⁵ The University of Texas at Austin 2515 Speedway Blvd Stop C1400 Austin TX 78712, USA
²⁶ Department of Astronomy and Astrophysics, University of California, Santa Cruz 1156 High Street Santa Cruz CA 95064, USA
²⁷ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM 91191 Gif-sur-Yvette, France
²⁸ Purple Mountain Observatory, Chinese Academy of Sciences 10 Yuanhua Road Nanjing 210023, China

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Received: 16 June 2025
Accepted: 14 November 2025

Abstract

Context. Clumps in the rest-frame UV emission of galaxies at z ≲ 3 have been observed for decades. Since the launch of the James Webb Space Telescope (JWST), a large population is detected in the rest-frame near-infrared (NIR), raising questions about their formation mechanism.

Aims. We investigate the presence and properties of NIR overdensities (hereafter substructures, including clumps) in star-forming and quiescent galaxies at 1 < z < 4 to understand their link to the evolution of their host galaxy.

Methods. We identified substructures in JWST/NIRCam F277W and F444W residual images at a rest-frame wavelength of 1 μm.

Results. The fraction of galaxies with substructures with M_★ > 10⁹ M_⊙ has steadily decreased with cosmic time from 40% at z = 4 to 10% at z = 1. NIR clumps, the most common type of small substructure, are much fainter (2% of the total galaxy flux) than similar UV clumps in the literature. Nearly all galaxies at the high-mass end of the main sequence (MS), starburst, and green valley regions have substructures. However, we do not find substructures in low-mass galaxies in the green valley and red sequence. Although massive galaxies on the MS and in the green valley have a 40% probability of hosting multiple clumps, the majority of clumpy galaxies host only a single clump.

Conclusions. The fraction of clumpy galaxies in the rest-frame NIR is determined by the stellar mass and star formation rate (SFR) of the host galaxies. Its evolution with redshift is due to galaxies moving toward lower SFRs at z ≲ 2 and the buildup of low-mass galaxies in the green valley and red sequence. Based on their spatial distribution in edge-on galaxies, we infer that most substructures are produced in situ via disk fragmentation. Galaxy mergers may still play a role at high stellar masses, especially at a low SFR.