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

TL;DR

This study probes metal enrichment during the Epoch of Reionization by linking neutral OI absorption at $z\sim6$ to nearby galaxies in the JWST EIGER survey. By combining a blind galaxy census of $[OIII]$ emitters with a quasar-absorption analysis, the authors identify five OI absorbers associated with galaxy overdensities and measure an OI covering fraction within $300~\mathrm{pkpc}$ of $0.27^{+0.13}_{-0.10}$, with absorbers extending to $R/R_{\rm vir}\sim8$. They estimate a lower-limit oxygen mass of $M_{\rm O}\gtrsim2\times10^{6} M_\odot$ within 300 kpc, comparable to the ISM O content of typical $z\sim6$ galaxies, implying substantial metals reside in extended environments. Relative abundances show elevated $[\mathrm{Si/O}]$ and $[\mathrm{C/O}]$ in galaxy-associated absorbers, often consistent with Population III enrichment, suggesting the imprint of first-generation stars in early cosmic structures. Overall, neutral OI at $z\sim6$ traces extended overdensity environments rather than individual halos, and its abundance patterns offer a window into Pop III-driven chemical enrichment during the early stages of galaxy evolution.

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. 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.

Figures (8)

• Figure 1: Redshift uncertainty distribution of the [O iii] emitter EIGER galaxies. The histogram shows the difference in redshift between emission-line measurements from NIRCam Grism Module A and Module B spectra, expressed in $\rm{km s^{-1}}$. Individual redshifts were derived from Gaussian fits to the [O iii] and H-$\beta$ lines. The two modules yield consistent results with a mean offset and standard deviation of $\Delta z = 0.0028 \pm 0.0073$ ($\Delta v = 9.3 \pm 24$$\rm{km s^{-1}}$).
• Figure 2: Properties of 40 spectroscopically confirmed galaxies at $5.9 \leq z \leq 6.4$ within 300 kpc of background quasars in the EIGER survey. Each square denotes a galaxy and is color-coded by redshift. Left: Angular positions and impact parameters relative to the background quasar (black star), with dashed concentric rings marking 100, 200, and 300 kpc at $z=6$. Right: Stellar mass versus star-formation rate, averaged over the last 50 Myr. Error bars show the 16th–84th percentile uncertainties in stellar mass and SFR.
• Figure 3: Voigt profile fits to O i and Si ii absorption profiles at $z=5.945$ toward the quasar SDSS J0100+2802. Shown are the flux (black), error spectrum (red), best-fit Voigt model (orange/black), and individual Voigt components (thin blue), using data from FIRE, X-Shooter, and HIRES. Vertical green lines mark galaxy members of the associated overdensity. Spectra from all instruments and all transitions are simultaneously fit using the rbvfit Bayesian absorption line fitter.
• Figure 4: Radial absorption profiles of O i absorption at $z \sim 6$. Each point marks the impact parameter to the nearest galaxy in a galaxy overdensity, with all detected O i absorption arising in such environments. Overdensities with O i absorption are shown as circles, while non-detections (2$\sigma$ limits) are indicated by downward arrows. Points are color-coded by the velocity offset between the galaxy redshift and the O i absorber, and their size scales with the number of members in the overdensity. The mean uncertainty on the virial radius is $\Delta R_{\mathrm{vir}} = 1.81~\mathrm{kpc}$. Top panels: Rest-frame equivalent width versus impact parameter ($R$, left) and normalized impact parameter ($R/R_{\mathrm{vir}}$, right). Bottom panels: Same as above, but for O i column densities.
• Figure 5: Galaxy overdensities around $z \sim 6$ O i absorbers in the EIGER survey. Each panel shows the background quasar (red star) at the center, with foreground galaxies associated with O i absorption marked as squares. Squares are color-coded by the velocity offset between the galaxy and the O i absorber. Only galaxies within 300 kpc of the quasar are shown, highlighting the overdensities nearest the absorbers. Dashed circles denote projected impact parameters of 50, 100, 200, and 300 pkpc from the quasar.