The dynamically relaxed galaxy cluster SPT-CL J2215-3537 is about 8.4 billion light-years from Earth, making it the farthest galaxy cluster ever spotted that is not being disrupted by violent collisions with other clusters of galaxies.
SPT-CL J2215-3537 resembles a blue cloud with a hazy golden light at its core. The cluster is set against the blackness of space, which is dotted with white, orange and blue specks of light. A combination of X-ray, ultraviolet, optical, and infrared light, was used to create the image. The blue cloud represents X-ray gas observed by NASA’s Chandra X-ray Observatory. The cloud is soft, and uniform in texture. Over time, galaxy clusters often merge or collide. These collisions cause disturbances and sharp features in a cluster’s gas. When a cluster takes on a smooth, calm appearance, as found here in SPT-CL J2215-3537, it indicates that no such collisions have occurred for a very long time. Astronomers call these galaxy clusters ‘relaxed.’ The hazy golden light at the heart of the cloud represents a massive central galaxy with a large amount of star formation. This central galaxy is quite isolated within the blue gas cloud. There are relatively few nearby specks of light near the central galaxy, and none are as bright or as large. No other large galaxies have been found within 600,000 light-years of the central galaxy. This implies that SPT-CL J2215-3537 has not experienced a galaxy merger in the last billion years. Image credit: NASA / CXC / MIT / M. Calzadilla / STScI / HST / N. Wolk.
Galaxy clusters are rich collections of hundreds to thousands of galaxies.
However, most of the luminous mass in a cluster is found in a hot X-ray-emitting phase that permeates the space between these galaxies.
Called the intracluster medium, this vast X-ray-emitting plasma can act as a tracer of what is happening to the cluster.
Galaxy clusters grow over time by merging with other galaxy clusters or groups, causing disturbances in the cluster’s gas such as asymmetries or sharp features.
Given enough time to ‘relax’ without a merger, however, this plasma can take on a smooth, calm appearance.
SPT-CL J2215-3537 (SPT2215 for short) is seen when the Universe is only 5.3 billion years old, compared to its current age of 13.8 billion years.
This implies that this 700-trillion-solar-mass 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.
“Up until now, we have not seen a relaxed galaxy cluster as distant as SPT2215,” said MIT astrophysicist Michael Calzadilla.
In the middle of the cluster resides SPT2215-BCG — the so-called central brightest cluster galaxy with a giant black hole in its center.
Calzadilla and his colleagues discovered huge amounts of new stars forming in this galaxy.
The formation of stars in a brightest cluster galaxy is fueled by the cooling of the hot gas when a cluster becomes relaxed.
How quickly the gas cools to form stars is influenced by the behavior of the giant black hole in the center of the cluster.
If the black hole drives too many powerful outbursts, then 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 in SPT2215-BCG does not appear to be preventing such cooling.
“It seems like the black hole in SPT2215-BCG is quiet enough to let star formation flourish,” said Dr. Michael McDonald, also of MIT.
Another 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.
This 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 astrophysicists were not sure they would find a galaxy cluster that was relaxed at this epoch of the Universe, because they are usually still undergoing the turmoil of mergers with other clusters or groups of galaxies as they increase in size.
“The fact that this cluster is so massive, so early in the universe suggests a really exciting, fast formation history,” said Dr. Lindsey Bleem, a researcher at Argonne National Laboratory.
“Yet the fact that it is relaxed suggests the opposite. It would be like finding a tidy kitchen right after the dinner rush.”
“Relaxed clusters like SPT2215 are one of the signposts that have been used to measure the expansion of the Universe,” said Dr. Adam Mantz, a researcher at Stanford University.
“Adding distant objects like this to our sample of relaxed clusters allows us to better constrain the acceleration of the cosmic expansion, and the properties of the dark energy that drives it.”
The discovery is reported in a paper in the Astrophysical Journal.
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Michael S. Calzadilla et al. 2023. SPT-CL J2215-3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster. ApJ 947, 44; doi: 10.3847/1538-4357/acc6c2
