'Unexpectedly calm' galaxy cluster discovered 8.4 billion light-years away from Earth

“The fact that this cluster is so massive, so early in the universe suggests a really exciting, fast formation history. Yet the fact that it is relaxed suggests the opposite. It would be like finding a tidy kitchen right after the dinner rush,” said Lindsey Bleem of the U.S. Department of Energy’s Argonne National Laboratory in Lemont, Illinois, whose team first reported spotting the cluster in 2020, in an official release. 

No signs of a merger in the last billion years in this cluster

Typically, galaxy clusters are made up of thousands to hundreds of galaxies that are held together by gravity. The space between each galaxy in the cluster is densely packed with hot gas and dark matter. 

Scientists are particularly interested in studying cosmic gas in order to figure out what is going on with the cluster. Mergers generate "disturbances in the cluster's gas, such as asymmetries or sharp features."

Galaxy clusters expand throughout time by merging with other galaxy clusters, and when no such mergers occur, they get ample time to "relax" in between. When this happens, the gas depicts a "calm appearance" — as seen in the SPT2215. 

The scientists discovered that the cluster had not merged with another cluster in the last billion years. And this identified cluster has a mass 700 trillion times more than the Sun.

“This implies that SPT2215 got a head start in its formation compared to other clusters of similar size and that it has been “coasting” for the last billion years, allowing it to relax,” noted the official release. 

Other notable features of the cluster 

The observations of this cluster were obtained using various facilities, including NASA’s Chandra X-ray Observatory, NASA’s retired Spitzer Space Telescope, the National Science Foundation/Department of Energy’s South Pole Telescope, and the Dark Energy Survey project in Chile.

The extensive observations from the advanced telescopes revealed a variety of additional characteristics of this cluster. 

Scientists discovered a huge galaxy in the middle of the cluster, which contains a massive black hole. They also found many new stars forming in this galaxy. 

The statement reveals that “how quickly the gas cools to form stars is influenced by the behavior of the giant black hole in the center of the cluster.” 

And it appears that this black hole is not very active, allowing new stars to develop. 

According to the scientists, researching these types of relaxed clusters might be useful in measuring the expansion of the universe. Additionally, the new results could reveal insights into the development and evolution of massive galaxy clusters found in the early galaxy. 

The results were published in a series of three papers in different journals.