Groups & Clusters of Galaxies

Deborah & William Hillyard

Science - Large Scale Structure of the Universe

This section looks at groups and clusters of galaxies in the Universe. This involves scales up to megaparsecs; that is, millions of light years. Let us consider our own Galaxy; the Milky Way. It is part of what is known as the Local Group of galaxies which comprises around 40 galaxies. The three largest galaxies in the Local Group are the Milky Way, Andromeda and Triangulum, which is much smaller than the other two. These three are the only spiral galaxies. About half of the remaining galaxies are satellites of the Milky Way and Andromeda. The group is approximately 3 Mpc in diameter (around 10 million light-years) and is shaped roughly like a dumbbell.

Generally, group is used to describe collections of up to 50 galaxies, and Cluster is used for collections of 50 to 1,000 individual galaxies, but this is not a strict rule. Groups are a few Mpcs in their largest dimension, while a cluster could be 10 Mpc or more across. Composition by mass is:
• approximately 5% visible matter; that is, stars and dust in the various galaxies
• 10% hot gas
• remainder dark matter.
But this can vary.

Groups & Clusters

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The proper motions of the galaxies in a group tends to be of the order of 100 miles/second, while in clusters, it is much greater; around 500 to 650 miles/second. This implies that the gravitation binding of the galaxies in a cluster is looser than it is in a group. Nevertheless, in both cases, outlying galaxies are much more susceptible to being pulled away than those closer to the center. In the same way that galaxies collide with each other, this also happens with groups and clusters. Generally, it seems that when this happens, the two components ultimately remain separate, passing through each other, but some galaxies would "change allegiance" in the process.

Here are descriptions of a few typical groups and clusters:

Virgo Cluster	A large cluster, Virgo contains about 1,500 galaxies and is about 18 Mpc away. It is in the Virgo Supercluster, which is also known as the Local Supercluster. It includes our own Local Group of galaxies. The cluster comprises three main collections of galaxies gathered around three large galaxies designated M87 (the largest collection), M86, and M49. As is usual in clusters, the elliptical galaxies tend to be in the inner part of the cluster.
Hercules Cluster	has about 300 galaxies and is about 200 Mpc (c. 650 Million light-years) away. It is in the Hercules Supercluster, which is itself contained in the Great Wall; see Filaments & Walls. It has many spiral galaxies, with a large number of interactions betweens its galaxies.
Coma Cluster	has over 1,000 galaxies and is about 100 Mpc away. In turn, it is in the Coma Supercluster, which contains more than 3,000 galaxies, and is in the central part of the Great Wall. Most of the galaxies are elliptical with two very large giant ellipticals NGC 4874 and NGC 4889. Here is a photograph of the cluster.
Norma Cluster	is hard to see as its position relative to the Milky Way means it is obscured by dust; in the so-call "zone of avoidance". It is in the middle of the Great Attractor.
Fornax Cluster	is a small cluster of about 58 galaxies located around 19 Mpc away.
Eridanus Group	contains around 200 galaxies is approximately 10 Mpc across, and about 23 Mpc away. This grouping is in the early stages of formation, and with the Fornax cluster, the Dorado group and others, forms the Eridanus-Fornax-Dorado Filament. It is very interesting because although many of its constituent galaxies are quite old, the group itself is rather young and still forming. It also has a large number of lenticular (type S0) galaxies. These are believed to be part of the evolution of spiral galaxies again implying great age for many of the galaxies.
The Abell 370 Cluster	a large cluster about 6 billion light years away. It comprises several hundred galaxies, and is the most distant cluster catalogued by George Abell. In the Image of the galaxy (see below), you can see arcs of light, which are the result of gravitational lensing by Abell 370 of more distant objects. One of them is the galaxy HCM-6A, which, at around 12.8 billion light years away, is one of the furthest known galaxies. The center image below shows the cluster, while the two outer images are details of the indicated parts of the image showing some of the gravitationally lensed galaxies behind the cluster. Click to expand any of the images. Clicking on the boxes or arrows will take you to the appropriate detailed image.