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Arms in the gas of the Coma cluster

Center of the Coma galaxy cluster showing the optical light and excess X-ray emission (red) overlayed. Unexpectedly, the X-ray emission shows long streams of hot X-ray emitting material, with the bottom left two streams apparently stripped gas from a group of galaxies which is merging with the main cluster. The optical light is dominated by a star to the top left and the brightest two galaxies in Coma, NGC 4889 (left) and NGC 4874 (right). This image measures 2.7 million light years across.
[Image courtesy of J.S. Sanders et al. / Sloan Digital Sky Survey]

X-ray readings from the center of one of the nearest galaxy clusters, the Coma cluster, suggest that the clashing of gas particles expected throughout the cluster interior doesn't happen in the very center, a new study in the journal Science reports. There, the gas calms down enough to allow particles brought to the core from other galaxy clusters to stay intact.

Galaxy clusters are structures consisting of hundreds to thousands of galaxies and all the turbulent, X-ray emitting gas in-between. Merging with other galaxies is one method by which they grow.

The Coma cluster is one of the clusters scientists have studied most, in part because it's relatively close to Earth. Galaxy subgroups outside the Coma cluster structure continue to merge with the main cluster on a regular basis, and this is how its massive size increases.

To understand more about these merging activities, and how they affect the Coma cluster's shape and size, J.S. Sanders (from Germany's Max Planck Institute for Extraterrestrial Physics) and colleagues used observations from NASA's Chandra X-ray Observatory to analyze X-ray emissions at Coma's core. They discovered several X-ray readings that were particularly bright, and also linear. The researchers liken them to "arms."

The strongest of these are bright arms of X-ray emissions extending from the center of the cluster towards a small galaxy subgroup called NGC 4911. The arms' characteristics, including a higher density plasma, are distinct from the surrounding intracluster medium, suggesting the arms were "stripped" from a merger of Coma cluster with NGC 4911.

Given the estimated timing of the merger, the arms are likely to be 300 million years old, the researchers say. Their long lifetime in Coma, where turbulent mergers never stop, implies that strong and choppy motions of galaxy clusters can be suppressed in the core region.