Star formation quenching precedes morphological transformation in COSMOS-Web’s richest galaxy groups

¹ INAF Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11 e Via Bazzoni 2, 34100, Trieste, Italy
² IFPU: Institute for Fundamental Physics of the Universe, Via Beirut, 2, 34151, Trieste, Italy
³ Department of Computer Science, Aalto University, PO Box 15400, Espoo, FI-00076, Finland
⁴ Department of Physics, University of Helsinki, P. O. Box 64, FI-00014, Helsinki, Finland
⁵ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2, I-40129, Bologna, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129, Bologna, Italy
⁷ Zentrum für Astronomie, Universität Heidelberg, Philosophenweg 12, 69120, Heidelberg, Germany
⁸ ITP, Universität Heidelberg, Philosophenweg 16, 69120, Heidelberg, Germany
⁹ Department of Physics and Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA, 92521, USA
¹⁰ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY, 14623, USA
¹¹ INFN – Sezione di Bologna, Viale Berti Pichat 6/2, 40127, Bologna, Italy
¹² CEA, Université Paris-Saclay, Université Paris Cité, CNRS, AIM, 91191, Gif-sur-Yvette, France
¹³ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780, Bochum, Germany
¹⁴ The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX, 78712, USA
¹⁵ Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
¹⁶ Department of Physics, University of California, Santa Barbara, CA, 93106, USA
¹⁷ Cosmic Dawn Center (DAWN), Denmark
¹⁸ Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY, 40506, USA
¹⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA
²⁰ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago, 75014, Paris, France
²¹ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing, 210023, China
²² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
²³ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA, 95064, USA
²⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200, Copenhagen, Denmark
²⁵ University of Geneva, 24 rue du Général-Dufour, 1211, Genève 4, Switzerland
²⁶ Department of Astronomy, University of Washington, Seattle, WA, 98195, USA

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Received: 27 November 2025
Accepted: 26 January 2026

Abstract

We analyzed the 25 richest galaxy groups in COSMOS-Web across the redshift range z = 0.18–3.65, identified using the Adaptive Matched Identifier of Clustered Objects (AMICO) algorithm. The groups have about 20–30 galaxies with a high (> 75%) membership probability. Our study reveals both passive-density and active-density relations, with late-type galaxies (LTGs) preferring both higher central overdensities than early-type galaxies (ETGs) across all groups, and secondly many massive LTGs have colors typical for quiescent galaxies. We identify red sequences (RS) in five out of 25 galaxy groups, prominently established at z < 1, with early emergence in the RS locus up to z ∼ 2.2. Although LTGs outnumber ETGs, ETGs are quenched much more efficiently than LTGs up to z ∼ 3.5. This finding suggests that group environments represent a transitional phase where star formation quenching precedes morphological transformation, contrasting with the classical morphology–density relation observed in rich clusters. In the central regions of the identified groups, within a radius of ∼33″ (100 kpc) from the group centers, we identified 86 galaxies. Among them, 23 (∼27%) were classified as ETGs and 63 (∼73%) as LTGs. High-mass galaxies (M_★ > 10^10.5 M_⊙) undergo rapid, transformative quenching over ∼1 Gyr, becoming predominantly spheroidal ETGs, indicating that morphological transformation accelerates dramatically in the most massive systems during the epoch of peak cosmic star formation. Intermediate-mass galaxies (10⁹ < M_★/M_⊙ < 10^10.5) show mild quenching, while low-mass galaxies (M_★ < 10⁹ M_⊙) remain largely star-forming, with environmental processes gradually suppressing star formation without destroying disk structures, suggesting that environmental quenching in groups operates on longer timescales than mass quenching. Overall, mass-dependent quenching dominates at the high-mass end, while environment-driven quenching shapes lower-mass systems, highlighting the dual nature of galaxy evolution across cosmic time. The fraction of high−luminosity active galactic nuclei (HLAGN) for both group and field galaxies increases with redshift, peaking at z ∼ 2, with groups consistently showing a higher AGN fraction than field. We suggest that AGN feedback plays a partial role in the rapid cessation of star formation in high-mass galaxies, while mergers may contribute to triggering AGN activity.