Astronomers may have found an explanation for one of the biggest mysteries revealed by the James Webb Space Telescope (JWST): why so many galaxies in the early universe appear unexpectedly bright in ultraviolet light. The new study, posted to the arXiv preprint server on May 11, suggests that galaxies more than 13 billion years ago were filled with an unusual kind of dust produced directly by supernova explosions, which could help explain why galaxies appeared so bright.
Too bright, too early
When JWST turned toward the universe’s earliest epochs, astronomers expected distant galaxies to appear as faint, dusty smudges. Instead, they found many galaxies—existing less than 550 million years after the Big Bang—blazing with ultraviolet light far brighter than any model had predicted.
Leading galaxy models assume that young, star-forming galaxies would be shrouded in dust, which absorbs ultraviolet radiation before it escapes into space—a dimming effect astronomers call attenuation. Without that shroud, galaxies appear dramatically brighter. Several competing explanations, including violent bursts of star formation, unusually efficient stellar nurseries, hidden black holes, and the unusual behavior of dust in these galaxies, emerged. The paper notes that the last explanation has gained the most interest recently because it’s physically natural and consistent with observations.
A competing explanation involving dust suggested that intense stellar feedback would physically blast dust out of young galaxies entirely. But JWST and Atacama Large Millimeter/submillimeter Array (ALMA) observations identified galaxies known as Galaxies with Extremely Low Dust Attenuation (GELDAs) that are simultaneously gas-rich and nearly UV-transparent. Gas fractions in some were found to exceed 90%. If dust had been blown out by violent feedback, the gas should have gone with it. So, it must have been something else.
The invisible dust?
In mature galaxies, most dust accumulates gradually through grain growth—tiny particles sweeping up metals from surrounding gas over billions of years. But in young galaxies existing when the universe was less than half a billion years old, there wouldn’t be enough time for this. The dominant available dust factories are the explosive deaths of massive stars, which are known as supernovae.
However, supernova dust does not arrive intact. A pressure wave called a reverse shock bounces back through ejected material, shattering the smallest grains and reducing total dust mass significantly. What survives is dominated by large grains that are intrinsically transparent to ultraviolet light.
In this new study, researchers led by D. Burgarella of Laboratoire d’Astrophysique de Marseille dug more into the role of dust in early galaxies. The answer, according to them, lies in the dust opacity, which is how effectively the dust blocks light. They developed a framework combining three ingredients—the known optical properties of supernova-produced dust, how that opacity scales with a galaxy’s metal content, and the physical arrangement of stars and dust clouds within galaxies—and tested whether this combination could reproduce what JWST actually observed.
Stardust to the rescue
When the team applied their dust properties to simulated galaxy populations, the results matched JWST’s observed counts without the need for exotic stellar physics or unusually efficient star formation. Their “stardust” model also explained the emergence of GELDAs in the early universe and their relative scarcity in the local universe.
It preserved their gas reservoirs, linking low attenuation to the intrinsic low opacity of supernova dust arranged in a porous geometry that allows the light to leak through gaps. “These Galaxies with an Extremely Low Dust Attenuation (GELDAs) are naturally consistent with the stardust model, where SNe dominate dust production, and suppresses attenuation,” the researchers wrote in the paper.
The model also incorporated a transition between dust regimes. Below a critical metallicity of roughly one-tenth of the sun’s metal content, supernova dust dominates and the dimming is low. Above it, interstellar grain growth takes over and dimming rises. This threshold had been theorized and observed locally, but JWST’s GELDAs appear to represent the first time this transition has been observed in action at high redshift.
Ashes of the first stars
Finally, in the most extremely metal-poor galaxies in their sample, the dust may carry an even deeper significance. The team identified several candidate galaxies so primitive that their dust could be a direct relic of Population III stars. These stars are the universe’s very first stellar generation, formed from pure hydrogen and helium before any heavier elements existed. These stars have never been directly observed. But their supernovae would have produced exactly the kind of large-grain, low-opacity dust this framework requires, and the chemical signatures of that dust may be detectable in the attenuation properties, metallicities, and infrared emission.
“This scenario links the observed GELDA population to the earliest stages of galaxy evolution,” the researchers concluded, “and offers a coherent framework to interpret the emerging JWST view of the cosmic dawn.”
The researchers caution not to overstate their conclusions. While the framework matches the observed galaxy population convincingly, the precise properties of supernova dust in the early universe remain uncertain. As the authors acknowledge, “The grain population emerging from SNe at high redshift may vary significantly from galaxy to galaxy.” Future observations with JWST’s infrared instruments and the ALMA radio telescope will be helpful to pin down these properties more precisely.
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Publication details
D. Burgarella et al, Stardust Galaxies at z>9: A Dust-Origin Transition Behind the Excess of UV-Bright Galaxies, arXiv (2026). DOI: 10.48550/arxiv.2605.09829
Journal information:
arXiv
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Supernova dust may be behind one of JWST’s biggest puzzles (2026, May 18)
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