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Coma ClusterWhy Study the Coma Cluster?The Coma Cluster is an ideal subject for observation. Less than five percent of Abell clusters contain as many galaxies as Coma does, and none are as close to the Earth. Because Coma is not obscured by the dust clouds of our galaxy, the Milky Way, it can be seen without interference. Since the advent of X-ray imaging, several phenomena have been observed in galaxy clusters and continue to be studied in Coma and elsewhere. Missing Mass Before X-ray astronomy, the amount of mass that could be observed in a cluster was based solely on visible data appearing as galaxies. High-temperature gas clouds, which are detected using X-ray telescopes, fill the cluster. These clouds have as much mass as all the stars in all the galaxies in the cluster! The cluster also contains a large amount of mysterious dark matter, about ten times the amount in gas and galaxies. This matter has yet to be observed with any type of telescope. The gravitational pull of this dark matter in clusters holds the movement of member galaxies and gas in the cluster. By mapping the X-ray emissions of Coma's gas cloud, the distribution of the gas can be modeled and the mass of the dark matter can be estimated more accurately. With this information from Coma and other clusters, the mass density of the entire universe could be calculated. This would allow astronomers to predict whether or not the universe will continue to expand indefinitely or if it will eventually begin to contract. The giant galaxy clusters have been assembled through the collision and merger of smaller groups and clusters over billions of years. X-ray images of Coma show this process, which releases more energy in the form of heat than millions of supernovas. The Hubble Constant The distances to galaxy clusters are important pieces of information that are used to calculate the age and rate of expansion of the universe, the Hubble Constant. Note on cosmic distances. The Coma Cluster is far enough away from Earth that an estimate of the Hubble Constant made using its distance would be representative of a smooth expansion of the universe. However, Coma is also close enough that astronomers can use X-ray images to help more accurately estimate its distance. Clusters such as Coma scatter background microwave radiation in what is known as the Sunyaev-Zel'dovich (S-Z) effect. The magnitude of the effect depends on the diameter of the cluster, and can be used to get an estimate of the distance to the Coma cluster and the Hubble constant. With the help of improved microwave and X-ray observation equipment, this method was used by a group of Caltech astronomers. Their estimate of the Hubble constant was between 46 and 101 km/s/Mpc, consistent with other estimates, which give an age for the universe between ten and twenty billion years. |
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Revised: August 29, 2006
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