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Cosmology - Galaxy Clusters

Galaxies are never found alone. They are all a part of a local group which is then a part of a cluster of galaxies. The role of Dark Matter plays a vital role in the distribution of galaxy clusters, but that is covered later.

Clusters do have some categories:

  • Regular clusters
  • Irregular clusters

Each category have two sub-categories:

  • Rich clusters - clusters that contain 50 or more galaxies
  • Poor clusters - clusters that contain less than 50 galaxies

Galaxy clusters are classified based on a system proposed by astronomy George Abell and are documented in the Abell Catalog.

Regular clusters are large and symmetrical that have a diameter between 1 and 10 Mpc. The rich and poor categories are defined by the number of galaxies located within 1.5 Mpc from the center of the cluster. This is called the Abell radius.

A regular cluster has at its center the brightest member, and is typically a large elliptical galaxy. This is called the Brightest Cluster Galaxy, or BCG.

The Coma cluster - Abell 1656 (above image) - is an example of a rich regular cluster. Here the center of the cluster is shared between the two bright elliptical galaxies (the two bright objects at the center of the image).

This cluster is 7Mpc in diameter and is thought to contain almost 2000 galaxies. The bright object in the upper right is a star from our own galaxy.

Irregular clusters can also have a prominent BCG (and can sometimes be a giant elliptical galaxy) but are more disorganized in appearance. There is also no centralized concentration of galaxies near the center of the cluster.

The Virgo cluster (above image) is hosted by the elliptical galaxy M87. but notice the field is not rich on galaxies, and there is no even galaxy distribution.

Galaxy clusters can be young as well. The image below is of the Hercules cluster.

The lack of elliptical galaxies indicates this cluster is young. Even though there is no BCG, this is still considered a regular cluster - notice the even distribution of galaxies.

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