Witnessing the onset of Reionisation via Lyman-alpha emission at redshift 13

Cosmic Reionisation commenced 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 $\mathrm{Ly}\alpha$ , in addition to 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 (IGM) becomes increasingly ionised. Together with a very blue UV continuum, the $\mathrm{Ly}\alpha$ line indicates the galaxy is a prolific producer of ionising photons, a significant fraction of which may escape. This suggests it resides in an early reionised region preventing complete extinction of $\mathrm{Ly}\alpha$ , thus shedding new light on the nature of the earliest galaxies and the onset of Reionisation only $330\ \mathrm{Myr}$ after the Big Bang.

eprint arXiv:2408.16608