Subaru and Webb Join Forces to Measure How Light Escaped the Early Universe
Featured image: [Artist’s impression of escaping Lyman-alpha radiation from early galaxies during the Epoch of Reionization; credit: NAOJ/NASA/ESA]
A team of Japanese astronomers led by Shunta Shimizu of the University of Tokyo has achieved the first direct measurement of the Lyman-alpha (Lyα) escape fraction during the Epoch of Reionization, combining data from the Subaru Telescope on Mauna Kea and the James Webb Space Telescope. The result, published as a preprint submitted to the Monthly Notices of the Royal Astronomical Society, reveals that roughly 10 percent of ionizing photons escape from galaxies at a redshift of 6.2, when the universe was less than a billion years old.
The measurement matters because it addresses one of the most persistent open questions in cosmology: what powered cosmic reionization, the period when neutral hydrogen fog burned away and the universe became transparent to light.
A Clever Filter Trick
The team exploited a rare coincidence in filter wavelengths. JWST’s NIRCam F470N filter, centered at 4.7 micrometers, captures Hα emission from galaxies at a redshift of approximately 6.2. Subaru’s Hyper Suprime-Cam (HSC) NB872 filter, centered at 872 nanometers, captures Lyα emission from the exact same redshift. Together, the two filters form a unique narrow-band pair that lets astronomers measure both hydrogen recombination lines from the same galaxies at the same cosmic epoch.
Previous measurements of the Lyα escape fraction at high redshift relied on indirect methods or small samples. This study identified 84 Hα emitters (HAEs) in the JWST CEERS (Cosmic Evolution Early Release Science) field, of which 56 yielded reliable photometry in the Subaru NB872 band. Nineteen showed Lyα emission above the 2-sigma threshold.
Stacking the data with completeness weighting produced a median escape fraction of 0.106, with an asymmetric error of plus 0.066 and minus 0.044. In other words, roughly one in every ten Lyα photons produced inside these galaxies actually makes it out into the intergalactic medium.
Who Ionized the Universe?
The Epoch of Reionization, spanning roughly from redshift 10 to 6, is the period when the first stars and galaxies emitted enough ultraviolet radiation to strip electrons from neutral hydrogen across the cosmos. For years, conventional models assumed that only the faintest, lowest-mass galaxies produced enough ionizing photons per stellar mass to drive this transition. Brighter galaxies were thought to trap their Lyα photons inside dusty or gas-rich interstellar media.
The Shimizu et al. results challenge that picture. The median escape fraction of approximately 10 percent is in line with other recent measurements at similar redshifts, but the key finding is that these galaxies are relatively luminous Hα emitters, not ultra-faint dwarfs.
“If Lyα escape traces Lyman continuum leakage,” the authors write, “this may suggest that relatively luminous HAEs, rather than only the faintest galaxies, can provide an important contribution to the ionizing photon budget during the EoR.”
What Controls Escape?
The study also sheds light on why some galaxies let Lyα photons out while others trap them. The team found that the escape fraction correlates positively with Lyα equivalent width and negatively with the UV continuum slope (β) and the rest-frame UV size. Compact, blue galaxies with steep UV slopes leak more Lyα. Critically, there was no significant correlation with the global dust extinction parameter E(B-V) or with rest-frame optical size.
These results suggest that the galaxy-to-galaxy variation in Lyα escape is driven by compact, low-attenuation star-forming components visible in the UV, not by the overall dust content of the galaxy. A small, intensely star-forming region with little dust can punch a clear channel for Lyα photons even inside a larger, dustier galaxy.
A New Window on Reionization
The dual narrow-band technique pioneered in this work opens a new observational channel for the Epoch of Reionization. As JWST continues its survey programs and Subaru’s HSC keeps collecting deep imaging, the sample of HAEs with both Hα and Lyα measurements will grow, tightening constraints on the escape fraction and its dependence on galaxy properties.
The paper (arXiv:2607.08264) is submitted to MNRAS and lists 18 authors from Japanese institutions including the University of Tokyo, Kyoto University, Waseda University, and NAOJ.