Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Joris Witstok, Peter Jakobsen, Roberto Maiolino, Jakob M. Helton, Benjamin D. Johnson
| Summary:
$require{mediawiki-texvc}$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 Myr
old. Smooth turnovers of their UV continua have been interpreted as
damping-wing absorption of Lyman-$alpha$ (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 Ly$alpha$, in addition to a smooth turnover. We
observe an equivalent width of $text{EW}_mathrm{Lyalpha} > 40 , AA$ (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 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
Ly$alpha$, thus shedding new light on the nature of the earliest galaxies and
the onset of Reionisation only 330 Myr after the Big Bang.
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