EIGER. VIII. First stars signatures in the connection between O I absorption and galaxies in the epoch of reionization Higginson, Jack Bordoloi, Rongmon Simcoe, Robert A. Matthee, Jorryt J ; https://orcid.org/0000-0003-2871-127X Kashino, Daichi Mackenzie, Ruari Kramarenko, Ivan ; https://orcid.org/0000-0001-5346-6048 Lilly, Simon J. Eilers, Anna-Christina Naidu, Rohan P. Yue, Minghao ddc:520 We investigate the association between galaxies and neutral O I absorption systems at z ∼ 6, which trace metalenriched 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 0.27 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 × 106 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 NCII NOI ( / ) 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. IOP Publishing 2026 info:eu-repo/semantics/article doc-type:Article Article http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/23115 https://research-explorer.ista.ac.at/download/23115/23136 Higginson J, Bordoloi R, Simcoe RA, et al. EIGER. VIII. First stars signatures in the connection between O I absorption and galaxies in the epoch of reionization. <i>The Astrophysical Journal</i>. 2026;999(1). doi:<a href="https://doi.org/10.3847/1538-4357/ae3e81">10.3847/1538-4357/ae3e81</a> eng info:eu-repo/semantics/altIdentifier/doi/10.3847/1538-4357/ae3e81 info:eu-repo/semantics/altIdentifier/issn/0004-637X info:eu-repo/semantics/altIdentifier/e-issn/1538-4357 info:eu-repo/semantics/altIdentifier/arxiv/2510.05220 info:eu-repo/semantics/openAccess