The story begins with a quasar called SDSS J0100+2802, an ultraluminous beacon whose light has traveled more than twelve billion years to reach Earth. Because quasars are so luminous, their light passes through clouds of gas scattered across cosmic history, and each cloud imprints a barcode of absorption lines onto the spectrum. Along this particular sight line, astronomers had previously identified a metal absorber at redshift z = 5.945, corresponding to a time near the end of the epoch of reionization, when the first galaxies were burning away the fog of neutral hydrogen that filled the primordial cosmos. Remarkably, that absorber showed over-abundant carbon and silicon compared with solar values, a chemical pattern consistent with the enrichment expected from Population III stars rather than from later generations of ordinary stars.

Population III stars are the holy grail of early-Universe astronomy. Unlike every star formed since, they contained no metals, the astronomer’s term for any element heavier than helium, because the primordial gas from which they condensed had been chemically untouched since the Big Bang. Their supernova explosions seeded the cosmos with the first carbon, oxygen, silicon and iron, setting the stage for all subsequent star formation. Yet no Population III star has ever been directly observed. They are too distant, too faint, and too brief-lived for current telescopes to catch in the act of shining. Instead, astronomers hunt for their chemical ashes: gas clouds whose elemental abundance patterns bear the distinctive nucleosynthetic signature of primordial stellar explosions.

The problem has always been context. A metal absorber with an unusual abundance pattern is suggestive, but without knowing what galaxies live nearby, it is hard to say whether the gas truly represents pristine first-star enrichment or something more mundane. That is where the new study, led by Zihao Li of the Cosmic Dawn Center and the Niels Bohr Institute at the University of Copenhagen, makes its decisive contribution. The team turned to data from the James Webb Space Telescope, gathered through the EIGER survey and its companion program ASPIRE, which use JWST’s near-infrared capabilities to find galaxies emitting strongly in the [O III] line around bright quasars at redshifts above six.

What they found in the J0100+2802 field was unexpected. Clustered near the metal absorber’s redshift was an unusually metal-poor galaxy overdensity: a protocluster-like structure containing seventeen spectroscopically confirmed member galaxies. When the team measured the gas-phase metallicity of this system, they found a mean value of roughly three percent of the solar abundance. That figure is about 0.4 dex, or a factor of roughly two and a half, more metal-poor than other galaxies of the same epoch found in similarly overdense environments. In other words, this crowded cosmic city is paradoxically one of the most chemically primitive places yet observed at this redshift, as if its galaxies had somehow been slow to inherit the metals manufactured by earlier stellar generations.

This combination, a chemically immature environment sitting next to an absorber with a Population III-like abundance pattern, is precisely what theoretical models predict for regions where first-star formation lingered late. The team’s interpretation is that this less chemically evolved system provided favorable conditions for preserving the absorption signatures of Pop III enrichment. In denser, more metal-rich environments, subsequent generations of stars would have rapidly overwritten the primordial chemical signature, mixing in the products of ordinary supernovae and erasing the evidence. Here, in a pocket of the Universe that remained unusually poor in metals even as galaxies assembled around it, the ancient fingerprint survived.

To understand the physical conditions that allowed this late survival of first-star chemistry, the researchers modeled the connection between the absorber and the surrounding galaxies using a halo occupation distribution framework, fitting the cross-correlation between the Pop III absorber and the galaxy sample with Markov chain Monte Carlo methods. The analysis yielded a minimum dark matter halo mass of log(M_h,min/M_sun) = 10.68, with substantial uncertainty ranging from about 9 to 11.6. This value carries a profound implication: it supports the scenario of late-time Population III formation at the outskirts of atomic hydrogen cooling haloes. Atomic cooling haloes, with masses around ten to the eighth solar masses, are the threshold structures in which hydrogen gas can cool efficiently enough to form stars without relying on molecular hydrogen. The finding suggests that the outer fringes of such massive halos, shielded from the metal pollution and harsh radiation of their crowded interiors, could host pockets of primordial gas that continued to form first stars hundreds of millions of years after the original cosmic dawn.

The technical details of the abundance analysis reinforce the picture. The absorber’s over-abundance of carbon and silicon relative to iron-group elements matches the nucleosynthetic yields calculated for Population III supernovae, particularly the faint, low-energy explosions that theoretical work has long suggested would characterize many first-star deaths. Such faint supernovae eject their outer layers rich in carbon and lighter elements while allowing the iron core to fall back into the remnant, producing exactly the carbon-rich, iron-poor pattern seen in both ancient metal-poor stars in the Milky Way’s halo and in this distant absorber. The consistency between the absorber’s chemistry, the metal poverty of the surrounding galaxies, and the halo mass inference forms a coherent, mutually reinforcing case.

The discovery arrives amid a flurry of Population III hunting enabled by JWST. Other teams have reported candidate Pop III galaxies through strong helium emission lines, extremely blue ultraviolet colors, and near-pristine gas pockets near the galaxy GN-z11, while surveys have identified ultra-faint, chemically primitive star-forming systems in the reionization era. Simulations such as THESAN-ZOOM have predicted that Population III star formation should continue until the very end of reionization, hidden in low-density pockets that survive the general enrichment of the intergalactic medium. The new result provides the strongest environmental evidence yet for that prediction, anchoring an indirect chemical detection to a concrete, mapped galaxy structure rather than an isolated absorption line.

What makes the finding genuinely exciting for the field is that it converts a needle-in-a-haystack search into a targeted strategy. If first-star imprints survive preferentially in metal-poor galaxy overdensities at the outskirts of atomic cooling halos near the end of reionization, then astronomers now know where to look: not in the emptiest voids, but in the chemically lagging suburbs of early cosmic cities. Future surveys with JWST and the next generation of extremely large telescopes can prioritize such environments, cross-matching quasar absorption spectroscopy with deep galaxy redshift surveys to build a census of surviving primordial chemistry. Each additional system found this way will tighten constraints on the initial mass function of the first stars, the energetics of their supernovae, and the duration of the Population III era, questions that currently span enormous theoretical uncertainty. For now, the seventeen galaxies gathered around SDSS J0100+2802 stand as a rare window onto a time when the Universe was still learning to make the elements from which everything, including ourselves, would eventually be built.

Subject of Research: Chemical imprints of Population III stars preserved in a metal-poor galaxy overdensity at the end of reionization

Article Title: First-star imprints in a metal-poor galaxy overdensity near the end of reionization

Article References: Li, Z., Kakiichi, K., Christensen, L., Cai, Z., D’Odorico, V., Matthee, J., Kashino, D., Bordoloi, R., Mackenzie, R., Berg, T. A. M., Vanni, I., Salvadori, S., Venditti, A., Zhang, S., Bosman, S. E. I., Bañados, E., Davies, F. B., Fan, X., Jun, H. D., … Zhu, Y. (2026). First-star imprints in a metal-poor galaxy overdensity near the end of reionization. Nature Astronomy. https://doi.org/10.1038/s41550-026-02993-5

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02993-5

Keywords: Population III stars, James Webb Space Telescope, reionization, quasar absorption, metal-poor galaxies, galaxy overdensity, early Universe, chemical enrichment, dark matter halos, primordial star formation, EIGER survey, Nature Astronomy