2025NatAs...9.1240P

Stars in galaxies form when baryons radiatively cool down and fall into dark matter-dominated gravitational wells. Eventually, star formation quenches as gas is depleted and/or perturbed by feedback processes that prevent the gas from collapsing and condensing. Here we report spatially resolved spectroscopic observations, using the JWST/NIRSpec integral field unit, of a massive, quiescent galaxy (Jekyll) and its neighbourhood at redshift z = 3.714, when the Universe age was 10% of today's. Jekyll resides in a massive dark matter halo ( M _DM > 10¹² M _ ⊙ _ ) forming a galaxy pair with Hyde, which shows intense dust-enshrouded star formation (star-formation rate ∼300 M _ ⊙ _ yr^–1). We find large amounts of kinematically perturbed ionized and neutral gas in the circumgalactic medium around the pair. Despite this large gas reservoir, Jekyll, which formed a stellar mass of 10¹¹ M _ ⊙ _ in stars and chemically enriched early (first billion years of the Universe) and quickly (200-300 Myr), has remained quiescent for over 500 Myr. The properties of the gas found around the two galaxies are consistent with intense, active galactic nucleus-induced photoionization, or intense shocks. However, with the current data, no obscured or unobscured active galactic nucleus is detected in the central galaxy (Jekyll) nor in the very active star-forming galaxy (Hyde). Our study points to a closed-box model followed by preventive feedback to explain the formation and early quenching of massive galaxies in the first 2 Gyr of the Universe.