JWST spectroscopic confirmation of the Cosmic Gems arc at z = 9.625

Insights into the small-scale structure of a post-burst system

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 Centre 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 3700 San Martin Drive Baltimore MD 21218, USA
⁷ Niels Bohr Institute, University of Copenhagen Jagtvej 128 2200-N Copenhagen, Denmark
⁸ Cosmic Dawn Center (DAWN), Denmark
⁹ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574 Saint-Genis-Laval, France
¹⁰ Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University 3400 N Charles St. Baltimore MD 21218, USA
¹¹ Instituto de Alta Investigación, Universidad de Tarapacá Casilla 7D Arica, Chile
¹² Dipartimento di Fisica, Università degli Studi di Milano Via Celoria 16 I-20133 Milano, Italy
¹³ Max-Planck-Institut für Astrophysik Karl-Schwarzschild-Str. 1 D-85748 Garching, Germany
¹⁴ INAF – IASF Milano Via A. Corti 12 I-20133 Milano, Italy
¹⁵ University of Ljubljana, Faculty of Mathematics and Physics Jadranska ulica 19 SI-1000 Ljubljana, Slovenia
¹⁶ Instituto de Física de Cantabria (CSIC-UC) Avda. Los Castros s/n. 39005 Santander, Spain
¹⁷ Center for Astrophysics | Harvard & Smithsonian 60 Garden Street Cambridge MA 02138, USA
¹⁸ 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
¹⁹ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University 3-4-1 Okubo Shinjuku Tokyo 169-8555, Japan
²⁰ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto 50 St. George Street Toronto Ontario M5S 3H4, Canada
²¹ Dunlap Institute for Astronomy and Astrophysics 50 St. George Street Toronto Ontario M5S 3H4, Canada
²² Department of Astronomy, University of Maryland College Park 20742, USA
²³ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara Via Saragat 1 I-44122 Ferrara, Italy
²⁴ Astrophysics Science Division, Code 660, NASA Goddard Space Flight Center 8800 Greenbelt Rd. Greenbelt MD 20771, USA
²⁵ School of Earth and Space Exploration, Arizona State University Tempe AZ 85287-1404, USA
²⁶ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) 1800 Sherman Avenue Evanston IL 60201, USA
²⁷ Observational Astrophysics, Department of Physics and Astronomy, Uppsala University Box 516 SE-751 20 Uppsala, Sweden
²⁸ INAF Osservatorio Astronomico di Padova vicolo dell’Osservatorio 5 35122 Padova, Italy
²⁹ Department of Physics, Ben-Gurion University of the Negev P.O. Box 653 Be’er-Sheva 84105, Israel

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Received: 24 July 2025
Accepted: 31 October 2025

Abstract

We present JWST/NIRSpec integral field spectroscopy of the Cosmic Gems arc, strongly magnified by the galaxy cluster SPT-CL J0615−5746. Six-hour integration using NIRSpec prism spectroscopy (resolution R ≃ 30 − 300), covering the spectral range 0.8 − 5.3 μm, reveals a pronounced Lyα-continuum break at λ ≃ 1.3 μm, as well as weak optical Hβ and [O III] λ4959 emission lines at z = 9.625 ± 0.002, located in the reddest part of the spectrum (λ > 5.1 μm). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope (β ≤ −2.5, F_λ ∼ λ^β). Spectral fitting with Bagpipes suggests that the Cosmic Gems galaxy is in a post-starburst phase, making it the highest-redshift system currently observed in a mini-quenched state. Spatially resolved spectroscopy at tens of parsecs shows relatively uniform features across subcomponents of the arc. These findings align well with the physical properties previously derived from JWST/NIRCam photometry of the stellar clusters, now corroborated by spectroscopic evidence. In particular, five observed star clusters exhibit ages of 7 − 30 Myr. An updated lens model constrains the intrinsic sizes and masses of these clusters, confirming they are extremely compact and denser than typical star clusters in local star-forming galaxies (Σ_M★ = 10⁵ − 10⁶ M_⊙). Additionally, four compact stellar systems consistent with star clusters (≲10 pc) are identified along the extended tail of the arc. A sub-parsec line-emitting HII1.2ex region straddling the critical line, lacking a NIRCam counterpart, is also serendipitously detected. The Cosmic Gems arc thus offers a rare opportunity to investigate, at parsec scales, the aftermath of a star formation burst in the early Universe.