Astronomers have discovered the most distant galaxy cluster and it is the most ‘relaxed‘ the one they’ve seen so far.
This cluster of galaxies, called SPT-CL J2215-3537 (SPT2215 for short), is the most distant ever spotted that is undisturbed by violent collisions with other galaxy clusters.
SPT2215 lies about 8.4 billion light-years from Earth and is visible when the universe is only 5.3 billion years old, compared to its current age of 13.8 billion years.
This implies that SPT2215 has taken a step ahead in its formation compared to other clusters of similar size and has been “freewheeling” for a billion years, allowing it to relax.
Astronomers estimate that the cluster has a mass about 700 trillion times that of the Sun.
Galaxy clusters are collections of tens to hundreds of galaxies, along with large amounts of hot gas and dark matter filling the space between galaxies, all held together by gravity.
They grow over time by merging with other galaxy clusters, causing disturbances in the gas of the cluster. However, given enough time to “unwind” without melting, the gas can take on a smooth, calm appearance.
Teams of scientists used data from NASA’s Chandra X-ray Observatory, their old Spitzer Space Telescope, the National Science Foundation/Department of Energy’s South Pole Telescope, and the Dark Energy Survey project in Chile.
“So far, we haven’t seen a relaxed galaxy cluster as distant as SPT2215,” said Michael Calzadilla of the Massachusetts Institute of Technology (MIT), lead author of the most recent of the three papers, which confirmed the cluster was relaxed.
In the middle of SPT2215 is a large galaxy containing a giant black hole at its center. Calzadilla’s paper discovered huge amounts of new stars forming in this large galaxy. Star formation in the central galaxy of a cluster is fueled by cooling hot gas as a cluster expands.
The behavior of the giant black hole at the center of the cluster influences how quickly the gas cools to form stars.
If the black hole causes too many powerful explosions, most of the gas in the cluster is prevented from cooling enough to form a flood of new stars. Unlike most relaxed clusters observed with Chandra, the giant black hole of SPT2215 does not appear to prevent such cooling.
“It appears that the black hole in SPT2215 is quiet enough to allow star formation to thrive,” said Michael McDonald, co-author of the three studies.
Another key feature of SPT2215 is the isolation of its central galaxy. There are no other galaxies within 600,000 light-years that are as bright or expansive.
Thus, the cluster has not experienced a merger with another cluster for about the last billion years, providing another piece of evidence that SPT2215 is relaxed.
Scientists weren’t sure they’d find a relaxed cluster of galaxies at this time in the universe, because they’re usually still going through the hustle and bustle of mergers with other galaxy clusters or groups as they grow in size.
“The fact that this cluster is so massive, so early in the universe suggests a really exciting and rapid formation story,” said Lindsey Bleem of the US Department of Energy’s Argonne National Laboratory in Lemont, Illinois, whose team first reported spotting the cluster in 2020.
“Yet the fact that he is relaxed suggests otherwise. It would be like finding a tidy kitchen right after the dinner rush.
These results on SPT2215 fit well with those from NASA’s James Webb Space Telescope, which showed galaxies forming at a very young age.
“Relaxed clusters like SPT2215 are one of the indicators that have been used to measure the expansion of the universe,” said Adam Mantz of Stanford University, who first reported the relaxed status of SPT2215 using Chandra data in 2022.
“Adding distant objects like this to our sample of relaxed clusters allows us to better constrain the acceleration of cosmic expansion and the properties of the dark energy driving it.”