PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction

¹ Geneva Observatory, Department of Astronomy, University of Geneva, Chemin Pegasi 51, CH-1290 Versoix, Switzerland
² Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4 Torrejón de Ardoz, E-28850, Madrid, Spain
³ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Astronomy, Indiana University, 727 East Third Street, Bloomington, IN 47405, USA
⁵ Departamento de Física de la Tierra y Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
⁶ Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA, Leiden, The Netherlands
⁷ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁸ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700AV, Groningen, The Netherlands
⁹ Space Telescope Science Institute (STScI), 3700 San martin Drive, Baltimore, MD 21218, USA
¹⁰ Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
¹¹ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹² Dunlap Institute for Astronomy and Astrophysics, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹³ Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8582, Japan
¹⁴ Division of Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
¹⁵ Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
¹⁶ Department of Astronomy, University of Texas, Austin, TX 78712, USA
¹⁷ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
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 DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155A, 2200 Copenhagen, Denmark
²¹ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
²² Department of Astronomy, Stockholm University, Oscar Klein Centre, AlbaNova University Centre, 106 91 Stockholm, Sweden
²³ AURA for the European Space Agency (ESA), Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
²⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

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Received: 2 February 2026
Accepted: 19 June 2026

Abstract

We present very deep (≈11 hours on-source) JWST/MIRI low-resolution spectroscopy of the rest-frame optical emission of U37126, a UV-bright (M_UV ≃ −20) mildly lensed (μ ≃ 2.2) galaxy at z = 10.255. The continuum emission is well detected in the NIRSpec and MIRI spectra, but no nebular recombination or metal emission lines are observed (EW₀ (Hβ+[O III]) ≤ 300 Å and EW₀ (Hα) ≤ 400 Å at 3σ). Combined with the exceptionally blue UV continuum slope, β_UV ≃ −2.9, and flat Balmer break, these constraints indicate a stellar population dominated by very young and massive stars with a strongly suppressed nebular contribution. Comparisons with synthetic stellar population models indicate that U37126 requires a very high ionizing photon production efficiency, log(ξ_ion/Hz erg⁻¹) ≃ 25.75, and a nearly unity Lyman continuum escape fraction, of f_esc ≥ 86% (3σ) based on the Hα flux limit and f_esc = 0.94 ± 0.06 derived independently from fitting the spectral energy distribution (SED). The best-fit SED yields a (delensed) stellar mass of M_★ ≃ 10^7.8 M_⊙ and a star formation rate SFR ≃ 10 M_⊙ yr⁻¹ (specific SFR ∼ 160 Gyr⁻¹). Together with its very compact size, r_eff ≃ 61 pc, this yields a very high stellar mass and SFR surface densities, ΣM_★ ≃ 3 × 10³ M_⊙ pc⁻² and Σ_SFR ≃ 400 M_⊙ yr⁻¹ kpc⁻². Together with the lack of detectable nebular emission, these properties suggest that U37126 is undergoing a so-called interstellar medium-naked starburst phase, possibly driven by an extremely efficient gas-to-star conversion followed by strong feedback that has cleared the remaining gas from its stellar core, which allowed most Lyman continuum photons to escape. Finally, we show that even a small fraction of galaxies such as U37126 (≃3–6%), with an extreme Lyman continuum production and escape, might contribute disproportionately (≃50–100%) to the ionizing photon budget during cosmic reionization.