The z = 9.625 Cosmic Gems galaxy was a compact “blue monster” propelled by massive star clusters^★

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna Via Gobetti 93/3 I-40129 Bologna, Italy
² Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Center SE-106 91 Stockholm, Sweden
³ Center for Frontier Science, Chiba University, 1-33 Yayoi-cho Inage-ku Chiba 263-8522, Japan
⁴ Department of Physics, Graduate School of Science, Chiba University, 1-33 Yayoi-Cho Inage-Ku Chiba 263-8522, Japan
⁵ Department of Astronomy, University of Michigan 1085 South University Avenue Ann Arbor MI 48109, USA
⁶ Space Telescope Science Institute (STScI) 3700 San Martin Drive Baltimore MD 21218, USA
⁷ Dipartimento di Fisica, Università degli Studi di Milano Via Celoria 16 I-20133 Milano, Italy
⁸ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574 Saint-Genis-Laval, France
⁹ Astrophysics Science Division, Code 660, NASA Goddard Space Flight Center 8800 Greenbelt Rd. Greenbelt MD 20771, USA
¹⁰ Department of Astronomy, University of Maryland College Park 20742, USA
¹¹ University of Ljubljana, Faculty of Mathematics and Physics Jadranska ulica 19 SI-1000 Ljubljana, Slovenia
¹² Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 D-85748 Garching, Germany
¹³ INAF – IASF Milano Via A. Corti 12 I-20133 Milano, Italy
¹⁴ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA
¹⁵ Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University 3400 N Charles St. Baltimore MD 21218, USA
¹⁶ Observational Astrophysics, Department of Physics and Astronomy, Uppsala University Box 516 SE-751 20 Uppsala, Sweden
¹⁷ Niels Bohr Institute, University of Copenhagen Jagtvej 128 DK-2200 Copenhagen N, Denmark
¹⁸ Cosmic Dawn Center (DAWN), University of Copenhagen 2200 Copenhagen N, Denmark
¹⁹ Instituto de Física de Cantabria (CSIC-UC) Avda. Los Castros s/n 39005 Santander, Spain
²⁰ Instituto de Alta Investigación, Universidad de Tarapacá Casilla 7D Arica, Chile
²¹ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) 1800 Sherman Avenue Evanston IL 60201, USA
²² David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto 50 St. George Street Toronto Ontario M5S 3H4, Canada
²³ Dunlap Institute for Astronomy and Astrophysics 50 St. George Street Toronto Ontario M5S 3H4, Canada
²⁴ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara Via Saragat 1 I-44122 Ferrara, Italy
²⁵ Department of Physics, Ben-Gurion University of the Negev P.O. Box 653 Be’er-Sheva 84105, Israel
²⁶ European Space Agency (ESA), European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villanueva de la Cañada Madrid, Spain
²⁷ Department of Astronomy/Steward Observatory, University of Arizona 933 N. Cherry Avenue Tucson AZ 85721, USA
²⁸ Department of Physics, School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo Shinjuku Tokyo 169-8555, Japan
²⁹ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo Shinjuku Tokyo 169-8555, Japan
³⁰ Instituto de Astrofísica de Andalucía–CSIC Glorieta de la Astronomía s/n E-18008 Granada, Spain
³¹ Observatório Nacional – MCTI (ON), Rua General José Cristino 77 São Cristóvão 20921-400 Rio de Janeiro, Brazil
³² School of Physics, University of Melbourne, Parkville Vic 3010, Australia
³³ Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba Tsukuba Ibaraki 305-8571, Japan
³⁴ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia

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Received: 23 July 2025
Accepted: 2 November 2025

Abstract

The recent discovery of five massive stellar clusters at z = 9.625 in the Cosmic Gems galaxy has raised the question about the formation mechanism of star clusters in the first 500 Myr after the Big Bang. We inferred the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM = n₀M^β), assuming three different slopes (β = −1.5, −2.0, and −2.5) and different lower-mass limits between 10² and 10⁵ M_⊙. We compared the total integrated cluster stellar mass to the stellar mass inferred from the counter image of the Cosmic Gems, which provides the best modestly magnified (μ = 1.84 ± 0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy was estimated as 3.5_−1.8^+3.3× 10⁷ M_⊙, with an effective radius of R_eff = 103₋₁₅⁺¹³ parsecs and a stellar surface mass density of Σ_mass = 520₋₂₂₅⁺³⁴⁰ M_⊙ pc⁻². Accounting for normalization uncertainties – including different lensing magnification scenarios for the arc – a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter image, without exceeding the galaxy’s stellar mass. By extrapolating the physical properties at the peak of the burst, we found that in its recent past (≲30 Myr) the Cosmic Gems galaxy likely experienced a specific star formation rate exceeding 25 Gyr⁻¹ and luminosity approaching the “blue monster” regime (M_UV < −20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and sheds light on the formation of bound star clusters that might survive to z = 0 as globular clusters older than 13 Gyr.