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

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.