Galaxy Clusters

March 25, 2009, 7:27 pm
Content Cover Image



On equivalent spatial scales, galaxy clusters are the largest perturbations to the cosmic matter density. Gravitational potential wells set by dark matter are populated with galaxies and intracluster baryons, including stars and gas.

General Characteristics

  • R~1-2 Mpc (Megaparsec) in radius.
  • M~1013-1015M_{sol} in mass.
  • There exist anywhere from two to 1000's of galaxies in a cluster.
  • The Milky Way resides in the Local Group which includes the Large and Small Magellanic Clouds.
  • Observable in a large range of wavelengths, including the optical, X-ray, and radio; these correspond with a suite of labeling parameters, which relate to the underlyling mass distribution; the characterization of the mass function.
  • Contain evolution of large-scale structure as tracers of the cosmic matter distribution.
  • Laboratories for testing of smaller-scale astrophysics.

Preview Image

This rich galaxy cluster, catalogued as Cl 0024+17, is allowing astronomers to probe the distribution of dark matter in space. The blue streaks near the center of the image are the smeared images of very distant galaxies that are not part of the cluster. The distant galaxies appear distorted because their light is being bent and magnified by the powerful gravity of Cl 0024+17, an effect called gravitational lensing. Dark matter cannot be seen because it does not shine or reflect light. Astronomers can only detect its influence by how its gravity affects light. By mapping the distorted light created by gravitational lensing, astronomers can trace how dark matter is distributed in the cluster. While mapping the dark matter, astronomers found a dark-matter ring near the cluster's center. The ring's discovery is among the strongest evidence that dark matter exists. This Hubble observation was taken in November, 2004 by the Advanced Camera for Surveys. (Source: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University))


Nord, Brian (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Galaxy Clusters." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 8, 2007].



(2009). Galaxy Clusters. Retrieved from


To add a comment, please Log In.