Astronomers have discovered the most distant “comfortable” galaxy cluster to date – the most distant cluster ever observed that has not been disrupted by violent collisions with other galaxy clusters. The discovery paves the way for studying how and when some of these giant structures formed and why the universe looks the way it does today.

To locate this distant and young galaxy cluster, teams of scientists used data from NASA’s Chandra X-ray Observatory, the surviving Spitzer Space Telescope, the National Science Foundation/Department of Energy South Pole Telescope, and the Dark Energy Research Project in Chile. They reported the results in a series of three articles.

Named SPT-CL J2215-3537 (abbreviated as SPT2215), this galaxy cluster is about 8.4 billion light-years from Earth and is only visible when it’s only 5.3 billion years old, compared to the current 13.8 billion-year age of the universe. This means that SPT2215 began its formation ahead of other clusters of similar size and has “moved” over the last billion years, allowing it to relax. According to astronomers, the mass of the cluster is about 700 trillion times greater than the mass of the Sun.

A galaxy cluster is a collection of tens to hundreds of galaxies, all held together by gravity, with large amounts of hot gas and dark matter filling the space between galaxies. This gas, which has a temperature of millions of degrees and emits X-rays, can serve as an indicator of what is happening to the cluster.

Galaxy clusters grow over time by merging with other galaxy clusters or groups, causing disturbances such as asymmetry or sharp features in the gas of the cluster. However, the gas can take on a smooth, calm appearance if given enough time to “relax” without coalescence.

“Until now, we’ve never seen a relaxed galaxy cluster like SPT2215 in the distance,” said Michael Calzadilla of the Massachusetts Institute of Technology (MIT), lead author of three recent papers that confirm the cluster is loose and report other important features of the cluster.

At the center of SPT2215 is a large galaxy with a supermassive black hole at its center. Calzadilla’s paper revealed that many new stars are forming in this massive galaxy. Star formation in the central galaxy of the cluster occurs due to the cooling of hot gas as the cluster relaxes.

The rate at which the gas cools to form stars is affected by the behavior of the giant black hole at the center of the cluster. If the black hole produces too many powerful flares, most of the gas in the cluster will not cool enough to form new stellar streams. Unlike most loose clusters observed with Chandra, the supermassive black hole in SPT2215 does not seem to prevent such cooling.

“Apparently the black hole in SPT2215 is quiet enough to allow star formation to flourish,” said Michael McDonald of MIT, co-author of all three papers.

Another important feature of SPT2215 is the isolation of its central galaxy. There is no other galaxy as bright or elongated as it within a radius of about 600,000 light-years. This means that the cluster has not undergone a merger with another cluster in the past billion years, further evidence that SPT2215 has been relaxed.

Scientists weren’t sure they would find a cluster of galaxies relaxed at this age of the universe, as they are often in the throes of merging with other galaxy clusters or groups as they grow in size.

“It’s really fascinating that this cluster is so large so early in the universe, it suggests a rapid formation date,” said Lindsay Blim of the U.S. Department of Energy’s Argonne National Laboratory in Lemont, Illinois, and her team first reported the cluster’s discovery in the first of three publications in 2020. “However, his relief suggests otherwise. It would be like finding a tidy kitchen right after dinner.”

These results in SPT2215 match up very well with results from NASA’s James Webb Space Telescope, which showed that galaxies formed at a very early age.

“Silent clusters like SPT2215 are one of the indicators used to measure the expansion of the Universe,” said Adam Manz of Stanford University, who first reported the relaxed state of SPT2215 in a second paper using Chandra data in 2022. “Adding such distant objects to our sample of relaxed clusters allows us to better constrain the acceleration of cosmic expansion and the properties of dark energy driving it.” Source