2024NatAs...8..657B

James Webb Space Telescope observations have spectroscopically confirmed the existence of galaxies as early as 300 Myr after the Big Bang and with a higher number density than what was expected based on galaxy formation models and Hubble Space Telescope observations. Yet, most sources confirmed spectroscopically so far in the first 500 Myr have rest-frame ultraviolet (UV) luminosities below the characteristic luminosity ($\({M}_\mathrm{UV}^{* }\)$), limiting the signal-to-noise ratio for investigating substructure. Here we present a high-resolution spectroscopic and spatially resolved study of a bright galaxy (M_UV = -21.66 ± 0.03, $\(\sim 2{M}_\mathrm{UV}^{* }\)$) at a redshift z = 9.3127 ± 0.0002 (510 Myr after the Big Bang) with an estimated stellar mass of $\(\left(1.6_{-0.4}^{+0.5}\right)\times 10^{9}\,M_{\odot }\)$, forming $\(1{9}_{-6}^{+5}\)$ solar masses per year and with a metallicity of about one tenth that of solar. The system has a morphology typically associated with two interacting galaxies, with a two-component main clump of very young stars (age less than 10 Myr) surrounded by an extended stellar population (120 ± 20 Myr old, identified from modelling the NIRSpec spectrum) and an elongated clumpy tidal tail. The observations acquired at high spectral resolution identify oxygen, neon and hydrogen emission lines, as well as the Lyman break, where there is evidence of substantial absorption of Lyα. The [O ii] doublet is resolved spectrally, enabling an estimate of the electron number density and ionization parameter of the interstellar medium and showing higher densities and ionization than in analogues at lower redshifts. We identify evidence of absorption lines (silicon, carbon and iron), with low confidence individual detections but a signal-to-noise ratio larger than 6 when stacked. These absorption features suggest that Lyα is damped by the interstellar and circumgalactic media. Our observations provide evidence of a rapid and efficient build-up of mass and metals in the immediate aftermath of the Big Bang through mergers, demonstrating that there were massive galaxies with several billion stars at early times.