A dense, metal-rich absorber driven by a heavily dust-obscured galaxy: The direct evidence of CGM enrichment at $z\sim7$

Qinyue Fei, Seiji Fujimoto, Gabriel Brammer, Lise Christensen, Marianne Vestergaard, Rohan P. Naidu, Robert A. Simcoe, Norman Murray, Luis C. Ho, Ruancun Li, Volker Bromm, Javier Álvarez-Márquez, Hollis B. Akins, Yoshihisa Asada, Simona Di Stefano, Kohei Inayoshi, Vasily Kokorev, Jorryt Matthee, Romain A. Meyer, Ava Polzin, Francesco Valentino, Fabian Walter, Marcel Neeleman, Yunjing Wu, Mengyuan Xiao

First listed 2026-09-17 | Last updated 2026-09-17

Abstract

We report the serendipitous discovery of a dense, metal-enriched circumgalactic medium (CGM) absorber at z=7.03, together with its host galaxy, identified along the sightline toward a red quasar. The absorber exhibits the largest rest-frame equivalent widths and column densities observed to date at z>5, tracing a metal-enriched gaseous halo. Using deep JWST/NIRSpec and NOEMA observations, we characterize the absorber's physical properties and identify its host, ND1, as a vigorously star-forming galaxy at an exceptionally close impact parameter of ~16kpc, so heavily dust-obscured that it is detected only in the longest-wavelength NIRCam bands. Detections of [OIII]$λ$5007, H$α$, and [CII]$λ158\,μ$m emission anchor the systemic redshift to z~7.0321, revealing a coherent ~50km/s blueshift of the absorbing gas relative to the host, suggesting outflow-driven metal transport into the halo. The outflow velocity, combined with the measured column density, implies an outflow rate of $\dot{M}_{\rm out}=64 M_\odot\,yr^{-1}$ and a mass loading factor $η=3$, indicating an energetic, chemically-enriched wind launched from a dust-obscured starburst. While the overall ionization state and [Mg/Fe] ratio align with empirical cosmic evolutionary trends, the absorber displays an enhanced carbon-to-oxygen ([C/O]) ratio. This abundance pattern suggests the nucleosynthetic yields of Population~III stellar explosions, and aligns with the host's star-formation history extending to $z \gtrsim 8$, potentially offering a rare fossil record of primordial metal enrichment. Our findings demonstrate that intense star-formation feedback can rapidly and efficiently pollute the CGM/IGM well into the Epoch of Reionization. The NIR-dark nature of the host further implies that rest-UV surveys may systematically miss the dusty sources that drive CGM- and IGM-scale metal enrichment in the early Universe.

Short digest

Fei et al. identify an exceptionally strong, dense metal absorber at z=7.03 along the sightline to the red quasar GNz7q and associate it with ND1, a vigorously star-forming, NIR-dark galaxy only about 16 pkpc away. JWST/NIRSpec detections of [O III] and Hα, together with NOEMA [C II], fix ND1’s systemic redshift and show the absorber is coherently blueshifted by about 50 km s⁻¹, supporting a chemically enriched outflow with an inferred mass-loss rate of 64 M⊙ yr⁻¹ and mass loading η=3. The system offers direct evidence that a dust-obscured starburst can enrich its CGM during reionization, while the absorber’s elevated [C/O] may preserve a signature of Population III enrichment. Its NIR-dark host also warns that rest-UV-selected galaxy samples can miss dusty sources responsible for early CGM and IGM metal pollution.

Key figures to inspect

• Figure 1. This establishes the observational foundation: the GNz7q spectrum contains the unusually rich, multiphase metal absorption system whose extreme equivalent widths and column densities motivate the absorber-host analysis.
• Figure 5. This is the key host-identification figure, combining ND1’s long-wavelength-only imaging detection, dust continuum context, and NIRSpec emission lines that anchor the galaxy at the absorber redshift.
• Figure 13. This directly tests the paper’s feedback interpretation by placing the absorption components against ND1’s emission-line systemic frame and visualizing the coherent blueshift attributed to an outflowing halo.
• Figure 14. This carries the chemical-enrichment conclusion: ND1’s inferred [C/O] and [Si/O] are compared with other high-redshift measurements and Population II versus Population III yield tracks, motivating the primordial-enrichment interpretation.