COSMOS-Web galaxy groups: Evolution of red sequence and quiescent galaxy fraction

¹ University of Bologna, Department of Physics and Astronomy “Augusto Righi” (DIFA) Via Gobetti 93/2 I-40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio Via Gobetti 93/3 I-40129 Bologna, Italy
³ Zentrum für Astronomie, Universität Heidelberg Philosophenweg 12 D-69120 Heidelberg, Germany
⁴ Institut für Theoretische Physik, Universität Heidelberg Philosophenweg 16 D-69120 Heidelberg, Germany
⁵ INFN – Sezione di Bologna Viale Berti Pichat 6/2 I-40127 Bologna, Italy
⁶ Department of Computer Science, Aalto University P.O. Box 15400 FI-00076 Espoo, Finland
⁷ Department of Physics, University of Helsinki P.O. Box 64 FI-00014 Helsinki, Finland
⁸ Department of Physics and Astronomy, University of California, Riverside 900 University Avenue Riverside CA 92521, USA
⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM 91191 Gif-sur-Yvette, France
¹⁰ Department of Astronomy, The University of Texas at Austin 2515 Speedway Blvd Stop C1400 Austin TX 78712, USA
¹¹ Aix Marseille Univ, CNRS, CNES, LAM Marseille, France
¹² Department of Physics, University of California, Santa Barbara Santa Barbara CA 93106, USA
¹³ Cosmic Dawn Center (DAWN), Denmark
¹⁴ Department of Physics and Astronomy, University of Hawaii, Hilo 200 W Kawili St Hilo HI 96720, USA
¹⁵ Caltech/IPAC, MS 314-6 1200 E. California Blvd. Pasadena CA 91125, USA
¹⁶ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology 84 Lomb Memorial Drive Rochester NY 14623, USA
¹⁷ DTU Space, Technical University of Denmark Elektrovej 327 2800 Kgs Lyngby, 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 Institute for Computational Cosmology, Department of Physics, Durham University South Road Durham DH1 3LE, United Kingdom
²¹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128 DK-2200 Copenhagen, Denmark
²² Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université 98 bis boulevard Arago F-75014 Paris, France
²³ Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive Pasadena CA 91001, USA
²⁴ Department of Physics and Astronomy, UCLA PAB 430 Portola Plaza Box 951547 Los Angeles CA 90095-1547, USA
²⁵ Department of Astronomy and Astrophysics, University of California, Santa Cruz 1156 High Street Santa Cruz CA 95064, USA
²⁶ National Research Institute of Astronomy and Geophysics (NRIAG) Cairo, Egypt
²⁷ University of Geneva 24 rue du Général-Dufour 1211 Genève 4, Switzerland
²⁸ University of Massachusetts Amherst 710 North Pleasant Street Amherst MA 01003-9305, USA

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Received: 10 September 2025
Accepted: 4 January 2026

Abstract

Aims. We investigate the redshift evolution and group richness dependence of the quiescent galaxy fraction and red sequence (RS) parameters in COSMOS galaxy groups, spanning a wide redshift range, from z = 0 to z = 3.7.

Methods. We analyzed the deep and well-characterized sample of groups recently detected with the AMICO algorithm in the COSMOS(-Web) field. Our study of the quiescent galaxy population is based on a machine-learning classification tool based on rest-frame magnitudes. The algorithm learns from several traditional methods to estimate the probability of a galaxy being quiescent, achieving high precision and recall. Starting from this classification, we computed quiescent galaxy fractions within groups via two methods: one based on the membership probabilities provided by AMICO, which rely on an analytical model, and another using a model-independent technique. We then detected the RS by estimating the ridgeline position using probability-weighted photometric data, followed by σ clipping to remove outliers. This analysis was performed using both rest-frame magnitudes and observer-frame magnitudes with rest-frame matching. We compared the results from both approaches and investigated the redshift and richness dependence of the RS parameters.

Results. We found that the quiescent galaxy population in groups builds up steadily from z = 1.5 − 2 across all richnesses, with faster and earlier growth in the richest groups. The first galaxies settle onto the RS ridgeline by z ∼ 2, consistent with current evolutionary scenarios. Notably, we reported a rare protocluster core hosting quiescent galaxies at z = 3.4, potentially one of the most distant early RSs observed. Extending our study to X-ray properties, we found that X-ray faint groups have, on average, lower quiescent fractions than X-ray bright ones, likely reflecting their typical location in filaments where pre-processing is lower. Leveraging the broad wavelength coverage of COSMOS2025, we traced RS evolution using observed and rest-frame colors over ∼12 Gyr, finding no significant trends in either the slope or the scatter of the ridgeline.