Breaking through the cosmic fog: JWST/NIRSpec constraints on ionizing photon escape in reionization-era galaxies

Aims. The escape fraction of Lyman continuum photons ( f esc (LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction ( f esc ) of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high f esc leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line and continuum emission, which also leads to bluer UV continuum slopes ( β UV ). Methods. In this work, we exploited the large archive of deep James Webb Space Telescope (JWST) NIRSpec spectra from the DAWN JWST archive to analyze over 1400 galaxies at 5 < z spec < 10 and constrain their f esc based on spectral-energy-distribution fitting enhanced with a picket-fence model. We identify 71 high-confidence sources with significant f esc based on Bayes-factor analysis strongly favoring f esc > 0 over f esc = 0 solutions. We compare the characteristics of this high-escape subset against both the parent sample and established diagnostics including β UV slope, O32, and SFR surface density (Σ SFR ). Results. For the overall sample, we find that most sources have a low escape fraction (< 1%); however, a small subset of sources seems to emit a large number of their ionizing photons into the IGM, such that the average f esc is found to be ∼10%, as needed for galaxies to drive reionization. Conclusions. Although uncertainties remain regarding recent burstiness and the intrinsic stellar ionizing-photon output at low metallicities, our results demonstrate the unique capability of JWST/NIRSpec to identify individual LyC leakers, measure average f esc , and thus constrain the drivers of cosmic reionization.