Witnessing the onset of reionization through Lyman-α emission at redshift 13

Cosmic reionization began when ultraviolet (UV) radiation produced in the first galaxies began illuminating the cold, neutral gas that filled the primordial Universe ^1,2. Recent James Webb Space Telescope (JWST) observations have shown that surprisingly UV-bright galaxies were in place beyond redshift z = 14, when the Universe was less than 300 Myr old ^{3, 4–5}. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-α (Ly-α), the principal hydrogen transition ^{6, 7, 8–9}. However, spectral signatures encoding crucial properties of these sources, such as their emergent radiation field, largely remain elusive. Here we report spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES ¹⁰) of a galaxy at redshift z = 13.0 that reveals a singular, bright emission line unambiguously identified as Ly-α, as well as a smooth turnover. We observe an equivalent width of EW _Ly-α > 40 Å (rest frame), previously only seen at z < 9 where the intervening intergalactic medium becomes increasingly ionized ¹¹. Together with an extremely blue UV continuum, the unexpected Ly-α emission indicates that the galaxy is a prolific producer and leaker of ionizing photons. This suggests that massive, hot stars or an active galactic nucleus have created an early reionized region to prevent complete extinction of Ly-α, thus shedding new light on the nature of the earliest galaxies and the onset of reionization only 330 Myr after the Big Bang.