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A study led by astronomers at the University of Arizona and published in Nature Astronomy shows that early galaxies were already enriching surrounding intergalactic space with heavy elements, including carbon, oxygen, and silicon, just 500 million years after the Big Bang
Leveraging near-infrared spectroscopy from NASA’s James Webb Space Telescope (JWST), the researchers observed evidence of “baryon cycling”, the process by which galaxies produce, expel, and recycle heavy elements, occurring far earlier in cosmic history than previously predicted.
Observational methodology: Galaxies as backlights
Standard cosmological models predicted that gas surrounding the first generations of galaxies consisted almost entirely of primordial hydrogen and helium.
To test this assumption, the research team analysed high-sensitivity, rest-frame ultraviolet spectra from three distant galaxies observed as they appeared during the Epoch of Reionisation, approximately 13 billion years ago.
The team used a distinct observational approach to detect the surrounding gas:
Background Illumination: Astronomers used the light from the galaxies themselves as backlights to probe the surrounding circumgalactic medium. As the light travelled Earthward, it passed through outward-flowing gas clouds, leaving subtle absorption signatures across specific elemental wavelengths.
Blueshifted Outflow Signatures: Analysis of nearly 30 hours of JWST spectral data revealed distinct blueshifted absorption features corresponding to oxygen, carbon, and silicon. The blueshift indicates that heavy elements forged inside stellar interiors were actively being pushed outward into intergalactic space at velocities ranging from 50 to 250 kilometres per second.
Implications for cosmic evolution and “Population III” stars
The finding that young galaxies were actively polluting their surroundings with heavy elements during the dawn of the universe reshapes fundamental models of galactic evolution:
Rapid baryon cycling:
The presence of heavy elements in circumgalactic gas shows that early galaxies were not isolated islands of star formation. Instead, they were part of an interconnected galactic ecosystem where enriched material was continuously exchanged.
The “Population III” star paradox:
Astronomers have long searched for Population III stars, hypothetical, pristine first-generation stars composed purely of primordial hydrogen and helium.
If galaxies began dispersing heavy elements throughout surrounding gas clouds within 500 million years of the Big Bang, truly pristine gas environments may have been extremely short-lived, offering a potential explanation for why Population III stars have eluded direct detection.
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