PRISMS: U37126, a very blue ISM-naked starburst at z = 10.255 with a nearly 100% Lyman continuum escape fraction Marques-Chaves, R. Álvarez-Márquez, J. Colina, L. Kendrew, S. Abdurro’Uf, Unknown Blanco-Prieto, C. Boogaard, L. A. Castellano, M. Caputi, K. I. Crespo-Gómez, A. Fontana, A. Fudamoto, Y. Fujimoto, S. García-Marín, M. Harikane, Y. Harish, S. Hashimoto, T. Hsiao, T. Iani, Edoardo ; https://orcid.org/0000-0001-8386-3546 Inoue, A. K. Langeroodi, D. Lin, R. Melinder, J. Napolitano, L. Östlin, G. Pérez-González, P. G. Prieto-Jiménez, C. Rinaldi, P. Rodríguez Del Pino, B. Santini, P. Sugahara, Y. Varo-O’Ferrall, A. Wright, G. Zavala, J. ddc:520 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. EDP Sciences 2026 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/22621 https://research-explorer.ista.ac.at/download/22621/22634 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> eng info:eu-repo/semantics/altIdentifier/doi/10.1051/0004-6361/202659281 info:eu-repo/semantics/altIdentifier/issn/0004-6361 info:eu-repo/semantics/altIdentifier/e-issn/1432-0746 info:eu-repo/semantics/altIdentifier/arxiv/2602.02322 info:eu-repo/semantics/openAccess