Galaxy groups and clusters are super-structures in the spread of galaxies of the cosmos. Matter throughout the visible Universe has, over the course of the Universe's history, aggregated into a range of large-scale structures under the influence of gravity. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are involved in larger groups called superclusters.

The galaxies of HCG 87, about four hundred million light-years distant. The large edge-on spiral, the fuzzy elliptical galaxy immediately to its right, and the spiral near the top of the image are members of the group, while the small spiral galaxy exactly in the middle is a more distant background galaxy.

Groups of galaxies

Groups of galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of about 2 megaparsec (Mpc) (see 1 E22 m for distance comparisons). Their mass are approximately 10¹³ solar masses. The spread of velocities for the individual galaxies is about 150 km/s.

The group which contains our own galaxy, the Milky Way, is called the Local Group, a group of more than 40 galaxies.

Clusters of galaxies

Clusters are larger than groups, although there is no sharp dividing line between a group and a cluster. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of an additional invisible mass component. X-ray studies have revealed the presence of large amounts of intergalactic gas. This gas is very hot, around 10⁸K, hence emits X-rays. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate equilibrium with the overall cluster gravitational field, its distribution in the cluster traces out the overall cluster gravitational field, and therefore allows calculation of the total mass distribution in the cluster. It turns out the total mass deduced from this measurement is much larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter.

Clusters typically have the following properties.

• They contain 50 to 1000 galaxies, hot X-ray emitting gas and large amounts of dark matter
• The distribution of these three components is approximately the same in the cluster.
• They have total masses of 10¹⁴ to 10¹⁵ solar masses.
• They typically have a diameter of 8Mpc (see 1 E23 m for distance comparisons).
• The spread of velocities for the individual galaxies is about 800-1000 km/s.
• The average distance between clusters is approximately 10 Mpc.

Note: clusters of galaxies should not be confused with star clusters such as galactic clusters and open clusters, which are structures within galaxies, as well as globular clusters, which typically orbit galaxies.

Superclusters

Main article: Supercluster

Groups, clusters and some isolated galaxies form even larger structures, the superclusters. At the very largest scales of the visible universe, matter is gathered into filaments and walls surrounding vast voids. This structure resembles a foam.

Related topics

• List of galaxy clusters
• Large-scale structure of the cosmos
• Timeline of galaxies, clusters of galaxies, and large-scale structure