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arXiv · 2506.13881

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

Wilfried Mercier·Boris Sindhu Kalita·Marko Shuntov·Rafael C. Arango-Toro·Olivier Ilbert·Laurence Tresse·Yohan Dubois·Clotilde Laigle·Hossein Hatamnia·Nicolas McMahon·Andreas Faisst·Isa Cox·Maxime Trebitsch·Leo Michel-Dansac·Si-Yue Yu·Michaela Hirschmann·Marc Huertas-Company·Arianna Long·Anton Koekemoer·Grégoire Aufort·Joseph Lewis·Ghassem Gozaliasl·R. Michael Rich·Jason Rhodes·Henry Joy McCracken·Caitlin Casey·Jeyhan Kartaltepe·Brant Robertson·Maximilien Franco·Daizhong Liu·Hollis Akins·Natalie Allen·Sune Toft
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Abstract

Clumps in the rest-frame UV emission of galaxies 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. We investigate the presence and properties of NIR over-densities (hereafter substructures) in star-forming and quiescent galaxies at 1 < z < 4 to understand their link to the evolution of their host galaxy. We identify substructures in JWST/NIRCam F277W and F444W residual images at a rest-frame wavelength of 1 um. The fraction of galaxies with substructures with M* > 10^9 Msun has been steadily decreasing with cosmic time from 40% at z = 4 to 10% at z = 1. Clumps, the main small substructures in the rest-frame NIR, are the most common type and are much fainter (2% of the 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. The fraction of clumpy galaxies in the rest-frame NIR is determined by the stellar mass and SFR of the host galaxies. Its evolution with redshift is due to galaxies moving towards lower SFRs at z < 2 and the build-up of low-mass galaxies in the green valley and red sequence. Based on their spatial distribution in edge-on galaxies, we infer that most of substructures are produced in-situ via disk fragmentation. Galaxy mergers may still play an important role at high stellar masses, especially at low SFR.

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Wilfried Mercier, Boris Sindhu Kalita, Marko Shuntov, Rafael C. Arango-Toro, Olivier Ilbert, Laurence Tresse, Yohan Dubois, Clotilde Laigle, Hossein Hatamnia, Nicolas McMahon, Andreas Faisst, Isa Cox, Maxime Trebitsch, Leo Michel-Dansac, Si-Yue Yu, Michaela Hirschmann, Marc Huertas-Company, Arianna Long, Anton Koekemoer, Grégoire Aufort, Joseph Lewis, Ghassem Gozaliasl, R. Michael Rich, Jason Rhodes, Henry Joy McCracken, Caitlin Casey, Jeyhan Kartaltepe, Brant Robertson, Maximilien Franco, Daizhong Liu, Hollis Akins, Natalie Allen, Sune Toft. 2025-06-16. Clumpiness of galaxies revealed in the near-infrared with COSMOS-Web. https://doi.org/10.1051/0004-6361%2F202555981

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