Long arms reach out of giant cluster
Unusual structures have been found in the central region of the Coma cluster, a large collection of thousands of galaxies located about 300 million light years from Earth. Observations with the X-ray satellites Chandra and XMM-Newton led by Jeremy Sanders from Max Planck Institute for Extraterrestrial Physics revealed remarkably long arms that tell astronomers about the collisions between Coma and other galaxy clusters over the last billion years.
The researchers think that these arms were most likely formed when the hot gas was stripped away from smaller galaxy clusters, while they were merging with the larger Coma cluster. This would have left a trail of superheated gas behind them similar to how a jet can leave behind contrails of water vapor as it moves across the sky.
Coma is an unusual galaxy cluster because it contains not one, but two giant elliptical galaxies near its center. These two giant elliptical galaxies are probably the vestiges from each of the two largest clusters that merged with Coma in the past. There are also other signs of past collisions and mergers that the researchers were able to uncover in the data.
The newly discovered X-ray arms are thought to be about 300 million years old, and they appear to have a rather smooth shape. This gives researchers some clues about the conditions of the hot gas in Coma. Most theoretical models expect that mergers between clusters like those in Coma will produce strong turbulence, like ocean water that has been churned by many passing ships. Instead, the smooth shape of these lengthy arms points to a rather calm setting for the hot gas in the Coma cluster, even after many mergers.
Two of the arms appear to be connected to a group of galaxies located about two million light years from the center of Coma. One or both of the arms connects to a larger structure seen in the XMM-Newton data, and spans a distance of at least 1.5 million light years. A very thin tail also appears behind one of the galaxies in Coma. This is probably evidence of gas being stripped from a single galaxy, in addition to the groups or clusters that have merged there.
Galaxy clusters are the largest objects held together by gravity in the universe. The collisions and mergers between galaxy clusters of similar mass are the most energetic events in the nearby universe. These new results are important for understanding the physics of these enormous objects and how they grow.
Large-scale magnetic fields are likely responsible for the small amount of turbulence that is present in Coma. Estimating the amount of turbulence in a galaxy cluster has been a challenging problem for astrophysicists. Researchers have found a range of answers, some of them conflicting, and so observations of other clusters are needed.