ALMA Detects Superheated Intracluster Gas in Distant Galaxy Protocluster
New observations of the young cluster SPT2349-56 with Atacama’s Large Millimeter/submillimeter Array (ALMA) have revealed unexpectedly hot intra-cluster gas just 1.4 billion years after the Big Bang, challenging current models of galaxy cluster evolution.
Artist’s impression of the forming galaxy cluster SPT2349-56: radio jets from active galaxies embedded in a hot intra-cluster atmosphere (red), illustrating a large thermal reservoir of gas in the nascent cluster. Image credit: Lingxiao Yuan.
SPT2349-56 is located about 12.4 billion light years away, meaning its light began reaching us when the Universe was only 1.4 billion years old, about a tenth of its current age.
The compact core of the protocol hosts several actively growing supermassive black holes and more than 30 star galaxies.
Galaxies form stars up to 1,000 times faster than our Milky Way and are crowded into a region of space about three times larger than the Milky Way.
“We didn’t expect to see such a hot cluster atmosphere so early in cosmic history,” said Dazhi Zhou, a Ph.D. candidate at the University of British Columbia.
Astronomers used an unusual observation technique called the thermal Sunyaev-Zel’dovich (tSZ) effect.
Rather than looking for light from the gas itself, this effect reveals a small shadow cast by hot electrons found in galaxy clusters on the faint afterglow of the Big Bang in the cosmic microwave background.
Before this new result, astronomers assumed that in early cosmic epochs, galaxy clusters were still too immature to have fully developed and heated their intra-cluster gas.
No hot cluster atmosphere had been directly detected during the first 3 billion years of cosmic history.
“SPT2349-56 changes everything we thought we understood,” said Professor Scott Chapman, a researcher at Dalhousie University and the University of British Columbia.
“Our measurements show a superheated cluster atmosphere only 1.4 billion years after the Big Bang, at a time when we thought intra-cluster gas should still be relatively cold and slowly settling.”
“This suggests that the birth of massive clusters could be much more violent and effective in heating the gas than our models assumed.”
According to the study, powerful explosions from SPT2349-56’s supermassive black holes, considered bright radio galaxies, could be a natural way to inject the enormous amount of energy needed to superheat intra-cluster gas so early.
This discovery suggests that during the first billion years of the Universe, energetic processes, such as bursts of supermassive black holes and intense bursts of stars, could significantly heat the surrounding gas in growing clusters.
This superheating stage could be crucial in transforming these young, cold galaxy clusters into the hot, sprawling clusters we see today.
It also suggests that current models need to update ideas about how galaxies and their environments develop.
This is the first direct detection of hot gas clusters ever reported, pushing the limits of astronomers’ ability to study these environments.
Discovering the existence of massive reservoirs of hot plasma so early is forcing scientists to rethink the sequence and speed of the evolution of galaxy clusters.
It also opens new questions about how supermassive black holes and galaxy formation shape the cosmos.
“SPT2349-56 is a very strange and exciting laboratory,” Zhou said.
“We see intense star formation, energetic supermassive black holes and this superheated atmosphere, all grouped together in a young, compact cluster.”
“There is still a huge observation gap between this violent first stage and the calmer clusters we will see later. »
“Mapping the evolution of their atmospheres over cosmic time will be a very interesting direction for future work. »
The results were published on January 5, 2026 in the journal Nature.
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D. Zhou and others. Sunyaev-Zeldovich detection of hot intra-cluster gas at redshift 4.3. Naturepublished online January 5, 2026; doi: 10.1038/s41586-025-09901-3
