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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.

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, and contains 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.
  

Notable galaxy clusters in the relatively nearby universe include the Virgo cluster and the Coma cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma cluster, is massive enough to affect the local expansion of the universe (Hubble flow).

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.

Finding Galaxy Clusters

Galaxy clusters are very difficult to find because:

• They are faint
  
• There are only a small number of them
  
• There are about 100 billion stars in our own galaxy, which get in the way of searches for fainter, more distant objects
  

Traditionally clusters of galaxies were found by looking through telescopes for groups of galaxies on the sky. This was quite inefficient and limited to nearby, bright clusters, so a number of new techniques have been developed:

• Infrared surveys -- distant clusters of galaxies tend to be redder than nearby stars, so it is slightly easier to spot them in the infrared
  
• Radio surveys -- very few objects in our own galaxy emit strong radio emission, so there is less of a problem with nearby sources getting in the way
  
• Sunyaev-Zeldovich effect surveys -- clusters of galaxies contain hot plasma (gas) which interacts with the cosmic microwave background radiation creating a sort of shadow against the background radiation at radio frequencies.
  
• X-ray surveys -- the hot plasma also emits X-rays which can be detected by X-ray telescopes.
  

Instruments and surveys for clusters of galaxies

• The AMI array at Mullard Radio Astronomy Observatory
  

Related topics

• List of galaxy clusters
  
• Large-scale structure of the cosmos
  
• Timeline of galaxies, clusters of galaxies, and large-scale structureca:Cúmul de galàxies
  

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