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Astronomers have discovered the most distant “relaxed” galaxy cluster to date — the farthest cluster ever spotted that is not being disrupted by violent collisions with other clusters of galaxies. The finding is paving the way to learning how and when some of these gigantic structures form and why the universe looks like it does in the present day.
This galaxy cluster, named SPT2215, is about 8.4 billion light-years from Earth and is viewed when the universe is 5.3 billion years old, compared to its current age of 13.8 billion years. That implies the cluster 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. Astronomers estimate the cluster has a mass some 700 trillion times that of the sun.
Galaxy clusters grow over time by merging with other galaxy clusters or groups, causing disturbances in the cluster’s gas. Given enough time to “relax” without a merger, however, the gas can take on a smooth, calm appearance.
One key feature of SPT2215 is the isolation of its central galaxy. There are no other galaxies within about 600,000 light-years that are anywhere near as bright or extended. That implies that the cluster has not experienced a merger with another cluster in about the last billion years, giving another piece of evidence that SPT2215 is relaxed.
The results on SPT2215 fit well with other observations, which have shown galaxies forming at a very young age. Relaxed clusters are signposts that have been used to measure the expansion of the universe.
To find this young, distant galaxy cluster, teams of scientists used data from the U.S. National Science Foundation’s South Pole Telescope Extended Cluster Survey, NASA’s Chandra X-ray Observatory, NASA’s retired Spitzer Space Telescope and the Dark Energy Survey project in Chile. The South Pole Telescope program is supported by NSF through grants 1248097 and 1852617. Partial support is also provided by an NSF Physics Frontier Center grant to the Kavli Institute of Cosmological Physics at the University of Chicago. Researcher A.A. Stark acknowledges additional NSF support.
The researchers reported the results in a series of three papers, published in The Astrophysical Journal, The Astrophysical Journal Supplement Series and the Monthly Notices of the Royal Astronomical Society.
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