| cluster of galaxies |
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A grouping of galaxies in space, linked by their mutual gravitational attraction. The distribution of galaxies in space is not uniform: they tend to clump together on distance scales of millions of light years. Clusters of galaxies take a variety of forms: they can be spherical and symmetrical, or ragged with no particular shape; they may contain a handful of galaxies or thousands; there may or may not be a concentration towards the centre. Regular clusters appear to be populated mainly by elliptical galaxies, while irregular clusters tend to include all galaxy types. Our own Milky Way Galaxy belongs to a small association known as the Local Group. Clusters containing a concentration of many large galaxies are described as "rich". The nearest rich cluster is the Virgo Cluster, which has thousands of members. Even larger is the Coma Cluster, which is at least ten million light years across. The centre of a rich cluster is typically dominated by a giant elliptical galaxy. The most massive galaxies known are at the centres of large rich clusters. It is believed that the largest galaxies tend to coalesce at the centres of clusters in a process called "galactic cannibalism". This is consistent with the observation that the second- and third-brightest galaxies in clusters of this sort are fainter than in clusters lacking an exceptionally bright galaxy. The cannibal galaxies are often distended in appearance, and evidence of more than one nucleus is seen in some. They are usually strong sources of radio emission. A tenuous hot gas pervades the space between the galaxies in rich clusters. Its presence has been discovered through its X-ray emission. In some cases, there is as much matter in the intergalactic gas as in the visible parts of the galaxies. Its presence tends to strip away interstellar gas from the spiral galaxies in the cluster. See also: galaxy, intracluster medium. |