JADES: Carbon enrichment 350 Myr after the Big Bang

Finding the emergence of the first metals in the early Universe and identifying their origin are some of the most important goals of modern astrophysics. We present deep JWST/NIRSpec spectroscopy of GS-z12, a galaxy at z = 12.5, in which we report the detection of the C iii]λλ1907,1909 nebular emission line. This represents the most distant detection of a metal transition, and the most distant redshift determination based on emission lines. In addition, we report tentative detections of [O ii]λλ3726,3729 and [Ne iii]λ3869, and possibly O iii]λλ1661,1666. By using the accurate redshift obtained from C iii], we can model the Lyα drop to reliably measure an absorbing column density of hydrogen of N_Hi ≈ 10²² cm⁻², which is too high for an IGM origin and implies an abundant neutral ISM in GS-z12 or in the CGM around it. We tentatively infer a lower limit for the neutral gas mass of about 10⁷ M which, compared with the galaxy stellar mass of ∼5 × 10⁷ M , implies a gas fraction higher than about 0.2–0.5. By comparing the measured emission lines with model-based diagnostic diagrams, we derive a solar or even super-solar carbon-to-oxygen ratio, tentatively log (C/O) > −0.21 dex ([C/O] > 0.15 dex), while a Bayesian modelling of the spectrum indicates log (C/O) = −0.30 ± 0.07 dex ([C/O] = 0.06 ± 0.07 dex). This is higher than the C/O measured in galaxies discovered by JWST at z = 6−9, and higher than the C/O arising from Type II supernovae enrichment. Asymptotic giant branch stars can hardly contribute to the observed carbon enrichment at these early epochs and low metallicities. Such a high C/O in a galaxy observed 350 Myr after the Big Bang may thus be explained by the yields of extremely metal-poor stars, and may even be the heritage of the first generation of supernovae from Population III progenitors. A robust determination of the total metallicity in this galaxy is essential to constrain these scenarios.