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 (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.