Galaxy groups and clusters
Galaxy groups and clusters are the largest
gravitationally-bound objects. They form the densest part of the
large scale structure of the Universe. In models for the gravitational formation of structure with
cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called
superclusters.
Groups of
galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of 1 to 2
Megaparsecs (Mpc) (see
1 E22 m for distance comparisons). Their mass are approximately 10
13 solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups.
The group which contains our own galaxy, the
Milky Way, is called the
Local Group, and contains more than 40 galaxies.
 |
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. |
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 either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the
intracluster medium. This gas is very hot, between 10
7K and 10
8K, 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 is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate
hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out 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 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 10
14 to 10
15 solar masses.
* They typically have a diameter of 2 to 10 Mpc (see
1 E23 m for distance comparisons).
* The spread of velocities for the individual galaxies is about 800-1000 km/s.
Notable galaxy clusters in the relatively nearby universe include the
Virgo cluster,
Hercules 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.
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.
Clusters of galaxies have been found in
surveys by a number of observational techniques and have been studied in detail using many methods:
*
Optical or
infrared: The individual galaxies of clusters can be studied through optical or infrared imaging and spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar
redshift. Infrared searches are more useful for finding more distant (higher
redshift) clusters.
*
X-ray: The hot plasma emits X-rays which can be detected by
X-ray telescopes. The cluster gas can be studied using both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along with
AGN are the brightest X-ray emitting extragalactic objects.
*
Radio: A number of
diffuse structures emitting at radio frequencies have been found in clusters. Groups of radio sources (which may include
diffuse structures or
AGN have been used as tracers of cluster location. At high
redshift imaging around individual radio sources (in this case
AGN) has been used to detect proto-clusters (clusters in the process of forming).
*
Sunyaev-Zel'dovich effect: The hot electrons in the intracluster medium scatter radiation from the
cosmic microwave background through
Compton scattering. This produces a "shadow" in the observed
cosmic microwave background at some radio frequencies.
*
Gravitational Lensing: Clusters of galaxies contain enough matter to distort the observed orientations of galaxies behind them. The observed distortions can be used to model the distribution of dark matter in the cluster.
Instruments and surveys used for finding clusters of galaxies
* The
AMI array, a
radio telescope at
Mullard Radio Astronomy Observatory* The
Sunyaev-Zel'dovic Array, a
radio telescope in California
*
XMM-Newton, an
X-ray telescope*
Chandra X-ray telescope.
*
Fossil group*
List of galaxy clusters*
Large-scale structure of the cosmos*
Timeline of galaxies, clusters of galaxies, and large-scale structure*
Intracluster medium
