First-star imprints in a metal-poor galaxy overdensity near the end of reionization

The first generation of stars, known as population III (Pop III), formed from primordial gas consisting solely of hydrogen and helium and is believed to have emerged only a few hundred million years after the Big Bang. Detecting the chemical enrichment of metal-poor circumgalactic gas offers a promising way to trace the enrichment signature of Pop III stars. Along the sight line to the quasar SDSS J0100+2802, a metal absorber at z = 5.945, showing over-abundant carbon and silicon compared with solar, has been reported to be consistent with the enrichment pattern of Pop III stars. Here, using data recorded by the James Webb Space Telescope, we report the discovery of an unusually metal-poor galaxy overdensity of 17 members (mean metallicity ~3% of solar) near this metal absorber, which is ~0.4 dex more metal-poor than coeval galaxies in similarly overdense environments. This less chemically evolved system may have provided favourable conditions for preserving the absorption signatures of Pop III enrichment. We infer a minimum dark matter halo mass of log(Mh,min/M⊙)=10.68−1.72+0.93, supporting late-time Pop III formation at the outskirts of atomic hydrogen cooling haloes. Our findings open a promising observational pathway to identifying the chemical imprints of the first stars and constrain the conditions for their formation.