Tour: Unexpectedly Calm and Remote Galaxy Cluster Discovered

Astronomers have discovered the most distant galaxy cluster with an important quality — 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.

To find this distant and unusually young galaxy cluster, teams of scientists used NASA’s Chandra X-ray Observatory along with NASA’s Spitzer Space Telescope, the South Pole Telescope, and the Dark Energy Survey project in Chile.

The cluster’s important quality is that it is “relaxed”, meaning that it is not being disrupted by violent collisions with other clusters of galaxies. This galaxy cluster, called SPT-CL J2215-3537 (or SPT2215 for short), is about 8.4 billion light-years from Earth. This means our telescopes see it when the universe is only 5.3 billion years old, compared to its current age of 13.8 billion years.

Astronomers think that galaxy clusters — enormous structures filled with individual galaxies, huge amounts of hot gas, and dark matter. In the case of SPT2215, researchers estimate that it has a mass some 700 trillion times that of the sun. Scientists think that galaxy clusters usually grow by merging with other clusters and smaller groups of galaxies over billions of years. This would have been especially true when the universe was younger. It was, therefore, surprising to find SPT2215 at its large distance from Earth. In other words, this discovery suggests that SPT2215 has become relaxed earlier than expected for a typical galaxy cluster.

Another interesting aspect of SPT2215 is the evidence for large amounts of star formation happening in its center. SPT2215 has a very large galaxy in its middle, which in turn has a supermassive black hole at its core. The prodigious amount of star formation shows scientists that much of the hot has cooled to the point where new stars can form, without outbursts driven by the black hole providing a heating source that prevents most of this cooling. This addresses an ongoing question of how much black holes stymie or support the birth of stars in their environments.

Relaxed clusters like SPT2215 are one of the signposts that have been used to measure the expansion of the universe. Adding distant objects like this to the sample of relaxed clusters allows astronomers to better constrain the acceleration of the cosmic expansion, and the properties of the dark energy that drives it.