EIGER VIII: First stars signatures in the connection between OI absorption and Galaxies in the Epoch of Reionization

Abstract

We investigate the association between galaxies and neutral OI 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 sightlines. Five OI absorption systems are associated with five of these overdensities, yielding a covering fraction of $0.27^{+0.13}_{-0.10}$ 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 CGM which is largely bound to halos. These galaxy-associated absorbers account for $\sim35\%$ of all OI systems seen in blind quasar surveys, implying the remainder arise in lower-mass galaxies below our detection threshold or in dense neutral IGM pockets. The CGM around these galaxies contains $\gtrsim 2\times10^6~M_{\odot}$ of oxygen, comparable to the ISM 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 $\log(N_{\rm CII}/N_{\rm OI})$ 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.