A massive interacting galaxy 525 million years after the Big Bang

Abstract JWST observations confirmed the existence of galaxies as early as 300 million years and with a higher number density than what was expected based on galaxy formation models and Hubble Space Telescope observations. Yet, sources confirmed spectroscopically so far in the first 500 million years have estimated stellar masses below 5×10 8 M ⊙ , limiting the signal to noise ratio for investigating substructure. Here, we present a high-resolution spectroscopic and spatially resolved study of a rare bright galaxy at a redshift z = 9.3127±0.0002 (525 million years after the Big Bang) with an estimated stellar mass of (2.5+ 0.7 −0.5 ) × 10 9 M ⊙ , forming 25 +3 −4 Solar masses per year and with a metallicity of about one tenth of Solar - lower than in the local universe for the stellar mass but in line with expectations of chemical enrichment in galaxies 1-2 billion years after the Big Bang. The system has a morphology typically associated to two interacting galaxies, with a two-component main clump of very young stars (age less than 10 million years) surrounded by an extended stellar population (130 ± 20 million years old, identified from modeling of 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 analogs at lower redshifts. For the first time at z > 8, we identify evidence of absorption lines (silicon, carbon and iron), with low confidence individual detections but signal-to-noise ratio larger than 6 when stacked. These absorption features suggest that Lyα is damped by the interstellar and circumgalactic medium. Our observations provide evidence of rapid and efficient built up of mass and metals in the immediate aftermath of the Big Bang through mergers, demonstrating that massive galaxies with several billion stars are present at earlier times than expected.