When the universe was still in its infancy—only 500 million years after the Big Bang, or about 3% of its current age—some of the universe’s earliest stars and galaxies had already formed. Astronomers have long predicted that much of the gas surrounding these young galaxies must have remained relatively pristine, composed mostly of hydrogen and helium, the primordial ingredients available in the newborn cosmos.
According to a study published by astronomers at the University of Arizona in Nature Astronomy, this picture is likely not correct. Instead, galaxies were already seeding the cosmos with heavy elements, such as oxygen and carbon, much earlier than astronomers expected.
“We observed that heavy elements escaped from galaxies very, very early in cosmic time,” said Yongda Zhu, first author of the paper and postdoctoral researcher at the U of A Department of Astronomy and Steward Observatory. “Not only were the galaxies producing these elements, but they were also dispersing them, possibly seeding other galaxies.”
How the universe got its elements
In the early universe, the cosmos barely contained anything other than hydrogen and helium, the two simplest elements at the very top of the periodic table. Over time, gravity pulled clouds of these simple elements together to form stars, where immense pressures and temperatures allowed nuclear fusion and other reactions to forge more complex, heavier elements such as carbon and oxygen.
When stars burned out or ended their lives as supernovae, they shed these heavy elements into space, where they became the building blocks for future generations of stars, planets and, ultimately, life. Even the carbon in our bodies and the oxygen we breathe were forged in earlier generations of stars.
Until now, however, it wasn’t clear how and when heavier elements were transported from the earliest galaxies into the surrounding universe.
Looking at early galaxies
Zhu’s research centered on three early galaxies whose light has traveled for more than 13 billion years, showing them as they appeared about 500 million years after the Big Bang, during a cosmic period known as the Epoch of Reionization.
At that time, the first generations of stars and galaxies were transforming the early universe by ionizing the hydrogen gas between them. This process, during which electrons were stripped from hydrogen nuclei, gradually brought an end to the cosmic “dark ages” by allowing ultraviolet light to travel more freely through the universe.
“We used the galaxies themselves as background light sources,” said Zhu. “As light from the galaxies traveled toward Earth, it passed through surrounding gas, and we were able to look at the light’s absorption patterns to detect specific elements.”
Observations of these ancient galaxies were made possible only by the infrared capabilities of NASA’s James Webb Space Telescope, which allowed researchers to observe galaxies as they appeared about 13 billion years ago. The nearly 30 hours of exposure provided enough light to detect faint absorption patterns in the spectra of these distant galaxies.
Over the course of one long night, Zhu manually searched through publicly available JWST spectra from hundreds of galaxies and pinpointed three with absorption patterns indicating the presence of heavy elements, including carbon, oxygen and silicon. The absorption lines were “blueshifted” relative to the galaxies’ redshift, indicating that the gas was moving outward from the galaxies and carrying oxygen, carbon and other heavy elements into intergalactic space.
The chemical fingerprints of these infant galaxies closely resembled those of evolved galaxies billions of years later, providing evidence that even at cosmic dawn, galaxies were already producing and spreading heavy elements into the space around them.
“Think of these elements, which originated from the galaxies’ stars, as food dye dropped into a cup of water,” said Zhu. “The color begins to spread through the water, and, in a similar fashion, these heavy elements from early galaxies began to escape into space and ‘enrich’ their surroundings.”
Baryon cycling
The process by which galaxies exchange material is known as baryon cycling. It is one reason galaxies are not isolated systems, but interconnected parts of a larger galactic ecosystem. Material produced by one generation of stars can be recycled and redistributed through this galactic ecosystem.
The discovery of early baryon cycling may also help explain why astronomers have struggled to find the first generation of stars, known as Population III stars. These stars are thought to have been the very first stars formed from pristine gas containing only hydrogen and helium, before heavier elements had been produced and dispersed throughout the universe.
If galaxies were already enriching their surroundings only 500 million years after the Big Bang, truly pristine gas—and the Population III stars that formed from it—simply may not have been around long enough to be observed.
“If you start out with pure vanilla ice cream but start mixing in sprinkles soon after, it won’t be long until you can no longer find any pristine, plain vanilla ice cream,” Zhu said.
Publication details
Early metal-enriched baryon cycling before the midpoint of cosmic reionization, Nature Astronomy (2026). DOI: 10.1038/s41550-026-02988-2
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University of Arizona
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JWST finds early galaxies were already seeding the universe with heavy elements (2026, September 24)
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