Groups and clusters of galaxies





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 of galaxies

Groups of galaxies are the smallest aggregates of galaxies. They typically
have the following properties.

   * They contain less than 50 galaxies.
   * They have a diameter of about 2 megaparsec (Mpc) (see 1 E22 m for
     distance comparisons).
   * Their mass is approximately 1013 solar masses.
   * The spread of velocities for the individual galaxies is about 150 km/s.

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
globular clusters and open clusters, which are structures within galaxies.

Superclusters

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.
See large-scale structure of the cosmos.

List of some close groups and clusters

   * Local Group
   * Maffei Group
   * Sculptor Group
   * M81 Group
   * NGC 5128 Group
   * Canes I Group
   * NGC1023 Group
   * M101 Group
   * Leo I Group
   * Canes II Group
   * Virgo cluster
   * Fornax cluster