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_ ,(̊m Hβ+ iii $)łeq 300$,Å and EW_ ,(̊m Hα)łeq 400,Å at 3σ). Combined with the exceptionally blue UV continuum slope, β_ ̊m 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_ ̊m ion / ̊m Hz, erg^ -1 ) ≃ 25.75, and a nearly unity Lyman continuum escape fraction, of f_ ̊m esc ≥ 86% (3σ) based on the Hα flux limit and f_ ̊m 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 mathrm SFR ≃ 10,M_⊙, (specific ̊m SFR ∼ 160,Gyr-1). Together with its very compact size, r_ yr^ -1 ̊m eff ≃ 61$,pc, this yields a very high stellar mass and SFR surface densities, Σ_ M⋆ ≃ 3 M_⊙,̊m pc^ 10^ 3 -2 and Σ_ ̊m SFR ≃ 400, M_⊙,̊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.