ASTRODEBATA / Joris Witstok: Witnessing the onset of Reionisation via Lyman-α emission in distant galaxies

Witnessing the onset of Reionisation via Lyman-α emission in distant galaxies

Joris Witstok
Cosmic Dawn Center (DAWN), Denmark

Cosmic Reionisation commenced when UV radiation produced in the first galaxies began illumi- nating the cold, neutral gas that filled the primordial Universe. Recent JWST observations have shown that surprisingly UV-bright galaxies were in place beyond redshift z = 14, when the Uni- verse was less than 300 Myr old. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-α (Lyα), the principal hydrogen transition. However, spectral signatures encoding crucial properties of these sources, such as their emergent radia- tion field, largely remain elusive. I will present NIRSpec observations from the JWST Advanced Deep Extragalactic Survey (JADES) allowing the characterisation of four newly discovered z > 8 galaxies exhibiting moderate or even strong Lyα emission. This was previously almost exclusively restricted to galaxies seen at z < 7 where the intervening intergalactic medium (IGM) becomes increasingly ionised, suggesting a perhaps unexpectedly early onset of Reionisation. Intriguingly, one galaxy at redshift z = 13.0 reveals a singular, bright emission line unambiguously identified as Lyα at a rest-frame equivalent width of EW > 40 Å. Together with an extremely blue UV continuum, the remarkably strong Lyα emission indicates the galaxy is a prolific producer and leaker of ionising photons. This suggests massive, hot stars or an active galactic nucleus have created an early reionised region to prevent complete extinction of Lyα, thus shedding new light on the nature of the earliest galaxies and their impact on Reionisation as early as 330 Myr after the Big Bang.

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