The technical ambition of MEGATRON lies in its simultaneous treatment of processes that are usually modelled separately. The simulations track the hydrodynamic motion of gas, the propagation of radiation emitted by young stars, the non-equilibrium chemistry of the cooling and fragmenting gas, and the synthesis and dispersal of heavy elements by supernova explosions. All of these are evolved self-consistently at exceptionally high resolution, following a single young galaxy that will eventually grow into a system comparable in mass to the Milky Way. That choice matters: rather than simulating hundreds of crude galaxies, the team resolved the internal structure of one progenitor deeply enough to capture the small-scale filaments and dense clumps where the first stars actually formed, structures that simpler models smear out entirely.

The simulations begin from pristine gas containing no heavy elements at all, mirroring the composition of the cosmos shortly after the Big Bang, when hydrogen and helium were essentially the only ingredients available. From those initial conditions, the code follows the birth of the first stars, the ultraviolet radiation they pour into their surroundings, the supernovae that end their brief lives, and the dispersal of newly forged carbon, oxygen and iron into the gas that will form subsequent generations of stars. Each stage feeds back into the next: radiation heats and ionises gas and suppresses fragmentation, supernovae drive metal-rich winds out of nascent galaxies, and the changing chemical composition alters how efficiently the gas can cool and form new stars. It is this coupled chain of events that determines the chemical fingerprints later recorded in long-lived low-mass stars.

One of the collaboration’s headline results concerns the smallest dwarf galaxies, the systems least able to hold onto their gas and metals. The published work examines how the first stars can create an iron metallicity plateau in these tiny systems, a regime where the iron abundance of successive stellar generations stops rising and instead flattens out. Understanding such plateaus is crucial for stellar archaeology, because astronomers infer the properties of the first stars from statistical patterns in the abundance ratios of ancient stars. If simplified models mispredict how metals are mixed, expelled and diluted in dwarf galaxies, the inferred properties of the first stellar generations could be systematically wrong. By resolving the interplay of radiation, gas flows and chemical enrichment at high resolution, MEGATRON provides a physically grounded calibration for interpreting those abundance patterns.

The broader conclusion of the first wave of results is a cautionary one for the field: simplified subgrid models may substantially underestimate the influence of stellar radiation and complex chemical processes on the gas surrounding early galaxies. Radiation from massive stars does not merely illuminate the gas; it heats it, ionises it, drives photoevaporative flows and regulates where subsequent stars can form. Chemical enrichment, meanwhile, does not proceed as a smooth, well-mixed process but through inhomogeneous, clumpy channels that leave distinct signatures in different environments. When these effects are modelled at high resolution, structures emerge in the circumgalactic gas that coarser simulations simply do not capture, and predictions for what JWST should observe change accordingly. Improving those predictions is one of the project’s central practical goals.

Dr Martin Rey of the Department of Physics at the University of Bath, a lead contributor to the collaboration, framed the significance of the work in terms of connection. “The James Webb Space Telescope gives us a direct glimpse of the infant cosmos, while stellar archaeology allows us to study the relics of those earliest times in our own Galactic neighbourhood,” he said. “MEGATRON provides a physical bridge between the two.” The phrase captures the project’s core claim: the same physical processes that JWST observes in action in distant young galaxies are the ones that wrote the chemical record now readable in ancient stars much closer to home.

That bridge also speaks to one of astronomy’s most fundamental questions: where the elements that make up today’s Universe came from. Every atom of carbon in DNA, every atom of oxygen breathed on Earth and every particle of iron in the planet’s core was forged inside a star and scattered into space by stellar death. “The elements that make our world and life possible – carbon, oxygen, iron and many others – were forged by stars,” Dr Rey explained. “To understand where those elements came from, we need to understand how the first stars formed and enriched their surroundings. MEGATRON allows us to test these ideas directly by comparing detailed simulations with observations from JWST and the chemical fingerprints preserved in ancient stars.” In this sense, the first stars are not merely a remote curiosity of the early Universe; they are the opening chapter of the chemical history that made rocky planets and life possible.

The timing of the project could hardly be better. JWST is transforming the observational picture of the earliest galaxies, revealing objects that are brighter, more numerous and more chemically evolved than many models had anticipated. At the same time, large-scale stellar surveys are cataloguing hundreds of thousands of ancient stars in the Milky Way and its dwarf satellites, measuring their abundance patterns with ever greater precision. Each dataset is powerful on its own, but the combination is uniquely constraining. “MEGATRON provides a common physical framework for interpreting two of astronomy’s most exciting new datasets: JWST’s view of the earliest galaxies and the stellar fossil record,” said Dr Rey. “Together, these complementary observations allow us to test competing models of the first stars in ways that weren’t previously possible.” A model of the first stars must now survive two independent tests at once: it must reproduce the galaxies JWST sees, and it must reproduce the abundance patterns written into the oldest stars.

The computational demands of this programme are enormous. The project has been awarded 40 million processor hours on the United Kingdom’s national supercomputers, a resource allocation the team compares to running five million laptops in parallel for a full year. Those hours are already being spent developing the next generation of MEGATRON simulations, which will push to higher resolution and incorporate more complete physical models, from improved treatments of radiative transfer to richer chemical networks. The payoff, the researchers argue, will be increasingly direct comparisons between simulated and observed galaxies, and between simulated and measured chemical abundance patterns, tightening the link between theory and data at every stage of cosmic history.

MEGATRON began in 2023 and is scheduled to run until 2030, with the four papers just published representing only the first substantial body of results and further releases expected to follow. If the collaboration’s central thesis holds, the coming years should see the two great archives of the cosmic dawn – the photons collected by JWST from the infant Universe and the chemical fossils preserved in the oldest stars of the Milky Way – converge into a single, mutually consistent account of how the first stars formed, died and seeded the cosmos with the elements from which everything, including us, is ultimately built.

Subject of Research: Cosmological simulations of the first stars and galaxies and their chemical enrichment of the early Universe

Article Title: New simulations connect the first stars to cosmic fingerprints still visible today

Article References: New simulations connect the first stars to cosmic fingerprints still visible today. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: first stars, early galaxies, MEGATRON simulations, James Webb Space Telescope, stellar archaeology, chemical enrichment, cosmic dawn, dwarf galaxies, supernovae, metallicity, University of Bath, computational astrophysics

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