The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST

In the first billion years after the Big Bang, sources of ultraviolet (UV) photons are believed to have ionized intergalactic hydrogen, rendering the Universe transparent to UV radiation. Galaxies brighter than the characteristic luminosity L* (refs. 1,2) do not provide enough ionizing photons to drive this cosmic reionization. Fainter galaxies are thought to dominate the photon budget; however, they are surrounded by neutral gas that prevents the escape of the Lyman-alpha photons, which has been the dominant way to identify them so far. JD1 was previously identified as a triply-imaged galaxy with a magnification factor of 13 provided by the foreground cluster Abell 2744 (ref. 3), and a photometric redshift of z approximate to 10. Here we report the spectroscopic confirmation of this very low luminosity (approximate to 0.05 L*) galaxy at z = 9.79, observed 480 Myr after the Big Bang, by means of the identification of the Lyman break and redward continuum, as well as multiple greater than or similar to 4 sigma emission lines, with the Near-InfraRed Spectrograph (NIRSpec) and Near-InfraRed Camera (NIRCam) instruments. The combination of the James Webb Space Telescope (JWST) and gravitational lensing shows that this ultra-faint galaxy (M-UV = -17.35)-with a luminosity typical of the sources responsible for cosmic reionization-has a compact (approximate to 150 pc) and complex morphology, low stellar mass (10(7.19) M circle dot) and subsolar (approximate to 0.6 Z(circle dot)) gas-phase metallicity.