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

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København N, DK-2200, Denmark
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen, Denmark
⁴ Max Planck Institut für Astrophysik, Karl Schwarzschild Straße 1, D-85741 Garching, Germany
⁵ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁷ Department of Astronomy & Astrophysics, University of Chicago, 5640 S Ellis Avenue, Chicago, IL 60637, USA
⁸ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
¹² Centro de Astrobiologí (CAB), CSIC-INTA, Ctra. de Ajalvir km4, Torrejón de Ardoz, E-28850 Madrid, Spain
¹³ LUX, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 75014 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 July 2025
Accepted: 26 January 2026

Abstract

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.