‘Why is this happening?’: Scientists discover large group of ancient galaxies that are too hot to exist

Astronomers have discovered an unexpectedly hot cluster of galaxies in the early universe, casting doubt on theories of galaxy evolution.

This scorching galaxy cluster existed just 1.4 billion years after the Big Bang, burning up much earlier and hotter than current models of cluster formation would predict. The findings suggest that predicted patterns of cluster growth may need to be reconsidered, the researchers reported in the journal Nature on January 5th.

A galaxy cluster is a collection of dark matter and hundreds to thousands of galaxies, all held together by gravity. These galaxies are separated by a mixture of gases known as the intracluster medium. When a galaxy cluster forms, the intracluster medium heats up due to gravitational interactions within the cluster and energy release from young stars and black holes. But this process takes a long time, and scientists have rarely observed hot intracluster media in young galaxy clusters.

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“Understanding galaxy clusters is key to understanding the largest galaxies in the universe,” study co-author Scott Chapman, an astrophysicist at Dalhousie University who conducted the study while at the National Research Council of Canada, said in a statement. “Most of these giant galaxies exist within galaxy clusters, and their evolution is greatly influenced by the extremely strong environment in which the clusters form, including the intracluster medium.”

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

Using a phenomenon called the thermal Sunyaev-Zeldovich effect, the researchers found that the gas in the intracluster medium is at least five times hotter than the gas at a younger age predicted by current theories of cluster formation.

“We did not expect to see such a hot star cluster atmosphere so early in the history of the universe,” study co-author Dazhi Zhou, a doctoral student in the Department of Physics and Astronomy at the University of British Columbia, said in a statement. “In fact, I was skeptical at first because the signal was too strong to be real.”

But it was reality. This may mean that galaxy clusters may form sooner than expected.

“This shows that something in the early universe, perhaps the three recently discovered supermassive black holes in this cluster, was already pumping massive amounts of energy into its surroundings, shaping young star clusters much faster and more strongly than we thought,” Chapman said.

In future work, the researchers plan to investigate what this unusual galaxy cluster means for the formation and evolution of existing galaxy clusters.

“We want to understand how intense star formation, active black holes, and this superheated atmosphere interact, and what that can tell us about how modern galaxy clusters were built,” Zhou said. “How can all of this happen at once in such a new and compact system?”