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.
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…
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