Brightest group galaxies in COSMOS-Web: Evolution of the size-mass relation since z = 3.7

¹ Department of Computer Science, Aalto University, PO Box 15400 Espoo FI-00076, Finland
² Department of Physics, University of Helsinki, P. O. Box 64 FI-00014 Helsinki, Finland
³ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
⁴ University of Bologna – Department of Physics and Astronomy “Augusto Righi” (DIFA), Via Gobetti 93/2 I-40129, Bologna, Italy
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3 40129 Bologna, Italy
⁶ Zentrum für Astronomie, Universität Heidelberg, Philosophenweg 12 69120 Heidelberg, Germany
⁷ Institute for Advanced Studies in Basic Sciences (IASBS), 444 Prof. Yousef Sobouti Blvd., Zanjan 45137-66731, Iran
⁸ Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁹ Department of Physics and Astronomy, University of Victoria, BC V8X 4M6 Canada
¹⁰ Infosys Visiting Chair Professor, Indian Institute of Science, Bangalore 560012, India
¹¹ Department of Physics, University of California, Santa Barbara, Santa Barbara, CA 93106, USA
¹² The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
¹³ Cosmic Dawn Center (DAWN), Denmark
¹⁴ Department of Physics, 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 Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁷ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom
¹⁸ Deutsches Zentrum für Astrophysik, Postplatz 1 02826 Görlitz, Germany
¹⁹ TU Dresden, Institute of Nuclear and Particle Physics, 01062, Dresden, Germany; DESY, Notkestrasse 85 22607 Hamburg, Germany
²⁰ Department of Physics and Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA
²¹ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
²² Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
²³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁴ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago 75014 Paris, France
²⁵ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
²⁶ DTU-Space, Technical University of Denmark, Elektrovej 327 2800 Kgs. Lyngby, Denmark
²⁷ ITP, Universität Heidelberg, Philosophenweg 16 69120 Heidelberg, Germany
²⁸ INFN – Sezione di Bologna, Viale Berti Pichat 6/2 40127 Bologna, Italy
²⁹ School of Physics and Astronomy, University of Southampton, Highfield SO17 1BJ, UK
³⁰ Department of Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
³¹ National Research Institute of Astronomy and Geophysics (NRIAG), Cairo, Egypt
³² EPFL Laboratory of Astrophysics (LASTRO), Observatoire de Sauverny, CH – 1290 Versoix, Switzerland
³³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91001, USA
³⁴ Astronomy Department, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
³⁵ Niels Bohr Institute, University of Copenhagen, Jagtvej 128 2200 Copenhagen, Denmark
³⁶ University of Geneva, 24 rue du Général-Dufour 1211 Genéve 4, Switzerland
³⁷ Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
³⁸ Thüringer Landessternwarte, Sternwarte 5 07778 Tautenburg, Germany
³⁹ Helmholtz-Institut für Strahlen-und Kernphysik (HISKP), Universität Bonn, Nussallee 14-16 D-53115, Bonn, Germany
⁴⁰ INAF-Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11 34143 Trieste, Italy
⁴¹ IFPU: Institute for Fundamental Physics of the Universe, Via Beirut, 2 34151, Italy

^⋆ Corresponding author: ghassem.gozaliasl@aalto.fi; natalie.allen@nbi.ku.dk

Received: 24 June 2025
Accepted: 5 September 2025

Abstract

We present the first comprehensive study of the structural evolution of brightest group galaxies (BGGs) from redshift z ≃ 0.08 to z = 3.7 using the James Webb Space Telescope’s 255-hour COSMOS-Web program. This survey provides deep NIRCam imaging in four filters (F115W, F150W, F277W, and F444W) in ∼ 0.54 deg², allowing robust size and morphological measurements for ∼ 1700 BGGs spanning ∼ 12 Gyr of cosmic history. High-resolution imaging enables consistent measurement of galaxy sizes in the rest-frame optical (red to near-infrared; ∼6000–8000 Å) across cosmic time through redshift-dependent filter selection. We classified BGGs as star-forming and quiescent using both rest-frame NUV–r–J colors and redshift-dependent specific star formation rate (sSFR) thresholds. Our structural analysis reveals that quiescent BGGs are systematically more compact than their star-forming counterparts across all redshifts, exhibiting steeper size–mass slopes (α_QG ∼ 0.6–1.2 vs. α_SF ∼ 0.0–0.3). The effective radius evolves as R_e ∝ (1+z)^−α, with α = 0.96 ± 0.07 for star-forming BGGs and α = 1.24 ± 0.09 for quiescent BGGs, indicating stronger size growth in quenched systems. The corresponding growth factor at fixed stellar mass (log M_∗ = 10.7) from z = 3.7 to z = 0.08 is ∼ 4.4 for star-forming and ∼ 6.6 for quiescent BGGs. The intrinsic scatter in the size–mass relation increases toward higher redshift for both populations, reaching ∼ 0.3–0.4 dex at z > 2, reflecting greater structural diversity in the early universe. Compared to field galaxies, BGGs show systematically smaller sizes at fixed stellar mass, particularly among quiescent systems, highlighting environmental effects on galaxy structure. We further compare the evolution of the quiescent fraction, the Sérsic index, and ellipticity with those of field galaxies, finding consistent trends that reinforce our main conclusions. These results establish the foundation for understanding how group-scale environments shape the structural evolution of central galaxies and provide crucial constraints for models of galaxy formation in intermediate-mass dark matter halos.