article published yes R. Marques-Chaves author J. Álvarez-Márquez author L. Colina author S. Kendrew author Unknown Abdurro’Uf author C. Blanco-Prieto author L. A. Boogaard author M. Castellano author K. I. Caputi author A. Crespo-Gómez author A. Fontana author Y. Fudamoto author S. Fujimoto author M. García-Marín author Y. Harikane author S. Harish author T. Hashimoto author T. Hsiao author Edoardo Iani author 4053390a-6b68-11ef-9828-a3b8adef8d0a0000-0001-8386-3546 A. K. Inoue author D. Langeroodi author R. Lin author J. Melinder author L. Napolitano author G. Östlin author P. G. Pérez-González author C. Prieto-Jiménez author P. Rinaldi author B. Rodríguez Del Pino author P. Santini author Y. Sugahara author A. Varo-O’Ferrall author G. Wright author J. Zavala author JoMa department We present very deep (≈11 hours on-source) JWST/MIRI low-resolution spectroscopy of the rest-frame optical emission of U37126, a UV-bright (MUV ≃ −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, βUV ≃ −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(ξion/Hz erg−1) ≃ 25.75, and a nearly unity Lyman continuum escape fraction, of fesc ≥ 86% (3σ) based on the Hα flux limit and fesc = 0.94 ± 0.06 derived independently from fitting the spectral energy distribution (SED). The best-fit SED yields a (delensed) stellar mass of M★ ≃ 107.8 M⊙ and a star formation rate SFR ≃ 10 M⊙ yr−1 (specific SFR ∼ 160 Gyr−1). Together with its very compact size, reff ≃ 61 pc, this yields a very high stellar mass and SFR surface densities, ΣM★ ≃ 3 × 103 M⊙ pc−2 and ΣSFR ≃ 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. https://research-explorer.ista.ac.at/download/22621/22634/2026_AstronomyAstrophysics_MarquesChaves.pdf application/pdfno EDP Sciences2026 eng 0004-6361 1432-0746 2602.0232210.1051/0004-6361/202659281 711 Marques-Chaves R, Álvarez-Márquez J, Colina L, Kendrew S, Abdurro’Uf U, Blanco-Prieto C, Boogaard LA, Castellano M, Caputi KI, Crespo-Gómez A, Fontana A, Fudamoto Y, Fujimoto S, García-Marín M, Harikane Y, Harish S, Hashimoto T, Hsiao T, Iani E, Inoue AK, Langeroodi D, Lin R, Melinder J, Napolitano L, Östlin G, Pérez-González PG, Prieto-Jiménez C, Rinaldi P, Rodríguez Del Pino B, Santini P, Sugahara Y, Varo-O’Ferrall A, Wright G, Zavala J. 2026. PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction. Astronomy and Astrophysics. 711, A301. Marques-Chaves, R., et al. “PRISMS: U37126, a Very Blue ISM-Naked Starburst at z = 10.255 with a Nearly 100% Lyman Continuum Escape Fraction.” <i>Astronomy and Astrophysics</i>, vol. 711, A301, EDP Sciences, 2026, doi:<a href="https://doi.org/10.1051/0004-6361/202659281">10.1051/0004-6361/202659281</a>. R. Marques-Chaves <i>et al.</i>, “PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction,” <i>Astronomy and Astrophysics</i>, vol. 711. EDP Sciences, 2026. Marques-Chaves, R., Álvarez-Márquez, J., Colina, L., Kendrew, S., Abdurro’Uf, U., Blanco-Prieto, C., … Zavala, J. (2026). PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction. <i>Astronomy and Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202659281">https://doi.org/10.1051/0004-6361/202659281</a> Marques-Chaves, R., J. Álvarez-Márquez, L. Colina, S. Kendrew, Unknown Abdurro’Uf, C. Blanco-Prieto, L. A. Boogaard, et al. “PRISMS: U37126, a Very Blue ISM-Naked Starburst at z = 10.255 with a Nearly 100% Lyman Continuum Escape Fraction.” <i>Astronomy and Astrophysics</i>. EDP Sciences, 2026. <a href="https://doi.org/10.1051/0004-6361/202659281">https://doi.org/10.1051/0004-6361/202659281</a>. Marques-Chaves R, Álvarez-Márquez J, Colina L, et al. PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction. <i>Astronomy and Astrophysics</i>. 2026;711. doi:<a href="https://doi.org/10.1051/0004-6361/202659281">10.1051/0004-6361/202659281</a> R. Marques-Chaves, J. Álvarez-Márquez, L. Colina, S. Kendrew, U. Abdurro’Uf, C. Blanco-Prieto, L.A. Boogaard, M. Castellano, K.I. Caputi, A. Crespo-Gómez, A. Fontana, Y. Fudamoto, S. Fujimoto, M. García-Marín, Y. Harikane, S. Harish, T. Hashimoto, T. Hsiao, E. Iani, A.K. Inoue, D. Langeroodi, R. Lin, J. Melinder, L. Napolitano, G. Östlin, P.G. Pérez-González, C. Prieto-Jiménez, P. Rinaldi, B. Rodríguez Del Pino, P. Santini, Y. Sugahara, A. Varo-O’Ferrall, G. Wright, J. Zavala, Astronomy and Astrophysics 711 (2026). 226212026-08-02T22:01:53Z2026-08-03T08:53:32Z