Galaxy Clusters

Most galaxies are found in groups or clusters which contain a few to thousands of galaxies bound together by their mutual gravity. These clusters contain not only galaxies, but also contain the material which lies between the galaxies. The space between the galaxies in a cluster is filled with hot gas as well as invisible dark matter which has not yet been identified. According to studies of how the galaxies in a cluster move (which is influenced by the total mass in the cluster), this dark matter could have about five times the mass of all the galaxies and hot gas in a cluster combined. Galaxy clusters are classified by their appearance as being either regular or irregular clusters. They are also classified as rich or poor clusters depending on the number of galaxies they contain.

The Coma Cluster of Galaxies ( O. Lopez-Cruz (INAOEP) et al., AURA, NOAO, NSF )
Regular Clusters

Regular clusters are spherically symmetric, with the galaxies concentrated toward the center of the cluster. They usually contain at least 1,000 galaxies which are brighter than an absolute magnitude of -16. Most of galaxies in regular clusters are elliptical galaxies. An example of a rich, regular cluster is the Coma Cluster (shown to the left), which contains thousands of galaxies. Almost every object in this image is a galaxy.

Irregular Clusters

Irregular clusters do not have a well-defined center, but are often made up of loose groups of small clusters. They contain all types of galaxies: spirals, irregulars and ellipticals. Irregular clusters can contain just a few to over 1,000 galaxies. The Local Group of galaxies, the cluster which contains our Milky Way galaxy, is an example of a poor, irregular cluster. Our Local Group contains about 30 galaxies: 3 spirals, 13 irregulars and 15 ellipticals.

The Fornax Irregular Cluster

A Galaxy Cluster showing Gravitational Lensing
Andrew Fruchter (STScI) et al., WFPC2, HST, NASA
Gravitational Lensing by Clusters

Einstein's general theory of relativity shows that a very large amount of mass can bend the path of light and warp spacetime. This effect is seen in many rich, massive clusters of galaxies. The huge mass of large clusters acts as a gravitational lens, bending light which enters the cluster from objects lying far behind it. This bent light is then focused by gravity to create one or more images of the light source. The image might be distorted, or even magnified by the cluster's gravitational lens, depending on the position of the light emitting source relative to the cluster and to the observer. Galaxy clusters usually create these images in the form of distended arcs. Cluster gravitational lenses allow us to observe objects that are much too far or too faint to be seen directly, helping us view the very distant, early universe.

The Spitzer Space Telescope will investigate the extent to which cooling flows of gas, gravitationally attracted to the cluster center, produces low-mass stars and brown dwarfs. Spitzer will also use the gravitational lensing effect of galaxy clusters to image distant galaxies that would otherwise be too faint to be seen.