Galaxy Clusters

Galaxies are usually found as members of clusters. Rich clusters can have thousands of members and poor clusters may have only dozens of galaxies. Galaxy clusters are the largest known stable structures in the universe.Clusters probably formed between 10 billion years ago and now. Groups and clusters may contain as many as ten to thousands of galaxies. The clusters themselves are often part of larger groups called superclusters. Galaxy clusters have been key astronomical objects in the development of our current understanding of a host of issues.

• Dark matter was first detected in galaxy clusters.
• Galaxy clusters contain gas at a temperature of about 108 K and are very luminous x-ray sources.
• The theory of gravitational instability, which is the current paradigm for the formation of all structure, was first developed to understand galaxy clusters.
• Clusters contain a unique mixture of galaxy types, including the biggest and many of the oldest.

Galaxy clusters were first tentatively recognized in the sky surveys of nebulae in the 19th century. The two most prominent in the northern sky are the Virgo cluster and the coma cluster, The Virgo cluster lies near the north Galactic pole and provides us with a convenient local sample of a relatively low-richness cluster. Coma is located in the north galactic cap and it stands out as an unusual structure in the whole northern extragalactic sky.
Clusters typically have the following properties.

• They contain as many as 50 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter.
• The distribution of these three constituents is approximately the same in the cluster.
• They have total masses of 1014 to 1015 solar masses.
• They typically have a diameter from 2 to 10 Mpc (Megaparsecs)
• The velocity range for the individual galaxies is about 800–1000 km/s.

Prominent galaxy clusters that we know of include the Virgo cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large assembly of galaxies known as the Great Attractor, dominated by the Norma cluster, is massive enough to affect the local expansion of the universe.
When observed visually, clusters appear to be agglomerates of galaxies held together by mutual gravitational attraction. However, their velocities are too high for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have uncovered the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, about107K and 108K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission. The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However this mass is still not enough to keep the galaxies in the cluster. Since this gas is almost in hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out that the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component has been called 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.

Questions to Ponder

• Will the diameter of a cluster of galaxies change with redshift?
• How can we discern different galaxies/clusters along the same line of sight?