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

Year: 2026

Type: article

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 ( EW 0 (H β +[O III ]) ≤ 300 Å and EW 0 (H α ) ≤ 400 Å at 3 σ ). 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% (3 σ ) based on the H α 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, Σ M ★ ≃ 3 × 10 3 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.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202659281

Institutions: University of Geneva, Consejo Superior de Investigaciones Científicas, Centro de Astrobiología, Space Telescope Science Institute, European Space Agency, Indiana University, Universidad Complutense de Madrid, Leiden University, Leiden Observatory, Astronomical Observatory of Rome, National Institute for Astrophysics, University of Groningen, Center for Frontier Science, Chiba University, University of Toronto, Institute for Cosmic Ray Research, The University of Tokyo, University of Tsukuba, Tomonaga Center for the History of the Universe, University of Tsukuba, The University of Texas at Austin, Institute of Science and Technology Austria, Waseda University, Waseda Research Institute for Science and Engineering, University of Copenhagen, University of Massachusetts Amherst, Stockholm University, AlbaNova, Royal Observatory, UK Astronomy Technology Centre

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $1,865

Funders: National Aeronautics and Space Administration, Villum Fonden, Space Telescope Science Institute, Ministerio de Ciencia e Innovación, Japan Society for the Promotion of Science, European Regional Development Fund, Agencia Estatal de Investigación

OpenAlex ID: W7165645822