'How can all of this be happening?': Scientists spot massive group of ancient galaxies so hot they shouldn't exist

Astronomers have spotted an unexpectedly hot galaxy cluster in the early universe that's challenging theories of galactic evolution.

The scorching cluster existed just 1.4 billion years after the Big Bang, blazing far earlier and hotter than current models of galaxy cluster formation predict should be possible. The discovery suggests that the predicted patterns of cluster growth might need a rethink, researchers reported Jan. 5 in the journal Nature.

Galaxy clusters are collections of dark matter and hundreds to thousands of galaxies, all bound together by gravity. Those galaxies are separated by a mix of gas known as the intracluster medium. As galaxy clusters form, the intracluster medium heats up due to gravitational interactions within the cluster and energy emissions from young stars and black holes. But this process takes a long time, and scientists have only rarely observed a hot intracluster medium in young galaxy clusters.

"Understanding galaxy clusters is the key to understanding the biggest galaxies in the universe," study co-author Scott Chapman, an astrophysicist at Dalhousie University who conducted the research while at the National Research Council of Canada (NRC), said in a statement. "These massive galaxies mostly reside in clusters, and their evolution is heavily shaped by the very strong environment of the clusters as they form, including the intracluster medium."

In the new study, researchers used the Atacama Large Millimeter/submillimeter Array (ALMA), a powerful radio telescope located in Chile, to observe a bright, young galaxy cluster known as SPT2349-56, whose light was emitted just 1.4 billion years after the Big Bang. This cluster is relatively small — about the size of the Milky Way's outer halo — but it contains more than 30 active galaxies and three supermassive black holes, and it forms stars more than 5,000 times as fast as the Milky Way.

Using a phenomenon called the thermal Sunyaev-Zeldovich effect, the team found that the gas in the intracluster medium is at least five times hotter than current theories of cluster formation predict it should be for its relatively young age.

"We didn't expect to see such a hot cluster atmosphere so early in cosmic history," study coauthor Dazhi Zhou, a PhD student in the department of physics and astronomy at the University of British Columbia, said in the statement. "In fact, at first I was skeptical about the signal as it was too strong to be real."

But it was real — and that could mean that galaxy clusters can form more quickly than expected.

"This tells us that something in the early universe, likely three recently discovered supermassive black holes in the cluster, were already pumping huge amounts of energy into the surroundings and shaping the young cluster, much earlier and more strongly than we thought," Chapman said.

In future studies, the team plans to investigate what this unusual cluster might mean for the formation and evolution of existing galaxy clusters.

"We want to figure out how the intense star formation, the active black holes and this overheated atmosphere interact, and what it tells us about how present galaxy clusters were built," Zhou said. "How can all of this be happening at once in such a young, compact system?"