EIGER. VIII. First Stars Signatures in the Connection between O I Absorption and Galaxies in the Epoch of Reionization

Abstract We investigate the association between galaxies and neutral O i absorption systems at z ∼ 6, which trace metal-enriched gas during the epoch of reionization. We identify 40 galaxies across six quasar fields, residing in 15 overdensities within 300 kpc of the background sight lines. Five O i absorption systems are associated with five of these overdensities, yielding a covering fraction of 0.2 7 − 0.10 + 0.13 within 300 kpc. The absorption occurs beyond typical virial radii, indicating that the gas traces extended overdensity environments rather than individual galaxy halos, unlike the z ∼ 0 circumgalactic medium (CGM), which is largely bound to halos. These galaxy-associated absorbers account for ∼35% of all O i systems seen in blind quasar surveys, implying the remainder arise in lower-mass galaxies below our detection threshold or in dense neutral intergalactic medium pockets. The CGM around these galaxies contains ≳ 2 × 10 6 M ⊙ of oxygen, comparable to the interstellar medium oxygen mass of the galaxies themselves, suggesting that the surrounding environment holds as much metal mass as the galaxies. All five galaxy-associated systems show significantly higher log ( N CII / N OI ) ratios than absorbers lacking galaxy associations. Furthermore, relative abundance ratios (Si/O, C/O) reveal that four of the five exhibit enrichment patterns consistent with Population III nucleosynthesis at the outskirts of galaxy overdensities. These rare systems offer a unique window into the role of first-generation stars in shaping the early metal enrichment of galaxies and their environments.