Witnessing the onset of reionization through Lyman-alpha 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. 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 $300\ \mathrm{Myr}$ old. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of $\mathrm{Lyman}-\alpha$ ($\mathrm{Ly}\alpha$ ), the principal hydrogen transition. 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) of a galaxy at redshift $z = 13.0$ that reveal a singular, bright emission line unambiguously identified as $\mathrm{Ly}\alpha$ , as well as a smooth turnover. We observe an equivalent width of $\mathrm{EW}_{\mathrm{Ly}\alpha} > 40\ \mathrm{\AA}$ (rest frame), previously only seen at $z < 9$ where the intervening intergalactic medium becomes increasingly ionized. Together with an extremely blue UV continuum, the unexpected $\mathrm{Ly}\alpha$ emission indicates that the galaxy is a prolific producer 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 $\mathrm{Ly}\alpha$ , thus shedding new light on the nature of the earliest galaxies and the onset of reionization only $330\ \mathrm{Myr}$ after the Big Bang.

Nature, Volume 639, Issue 8056, pages 897-901