Andrew Bunker

Abstract:

We present deep JWST/NIRSpec integral-field spectroscopy (IFS) and ALMA [C ii]158m observations of COS-3018, a star-forming galaxy at z 6.85, as part of the GA-NIFS programme. Both G395H (R 2700) and PRISM (R 100) NIRSpec observations revealed that COS-3018 is comprised of three separate components detected in [O iii]5007, which we dub as Main, North, and East, with stellar masses of 10, 10, 10 . We detect [O iii]5007,4959, [O ii]3727,3729, and multiple Balmer lines in all three components together with [O iii]4363 in the Main and North components. This allows us to measure an interstellar medium temperature of = 1.27 and = 1.6 K with densities of = 1250250 and = 700200 cm, respectively. These deep observations allow us to measure an average metallicity of 12 + log(O/H) = 7.9–8.2 for the three components with the T-method. We do not find any significant evidence of metallicity gradients between the components. Furthermore, we also detect [N ii]6585, one of the highest redshift detections of this emission line. We find that in a small, metal-poor clump 0.2 arcsec west of the North component, N/O is elevated compared to other regions, indicating that nitrogen enrichment originates from smaller substructures, possibly proto-globular clusters. [O iii]5007 kinematics show that this system is merging, which is probably driving the ongoing, luminous starburst.

Abstract:

Cosmic reionization began when ultraviolet (UV) radiation produced in the first galaxies began illuminating the cold, neutral gas that filled the primordial Universe1, 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 old3, 4–5. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-α (Ly-α), the principal hydrogen transition6, 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 (JADES10) 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 EWLy-α > 40 Å (rest frame), previously only seen at z < 9 where the intervening intergalactic medium becomes increasingly ionized11. 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.