A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014–1015 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.
Galaxy clusters typically have the following properties:
- They contain 100 to 1,000 galaxies, hot X-ray emitting gas and large amounts of dark matter.
- The distribution of the three components is approximately the same in the cluster.
- They have total masses of 1014 to 1015 solar masses.
- They typically have a diameter from 1 to 5 Mpc.
- The spread of velocities for the individual galaxies is about 800–1000 km/s.
One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster.
Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster (was first discovered) Fornax Cluster, Hercules Cluster, and the Coma Cluster.
Other galaxy clusters are:
Bullet Cluster- consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 1.141 Gpc (3.72 billion light-years). Gravitational lensing studies of the Bullet Cluster are claimed to provide the best evidence to date for the existence of dark matter.
Musket ball cluster- is a galaxy cluster that exhibits separation between its baryonic matter and dark matter components. The cluster is a recent merger of two galaxy clusters. It is named after the Bullet Cluster, as it is a slower collision, and older than the Bullet Cluster. This cluster is further along the process of merger than the Bullet Cluster, being some 500 million years older, at 700 million years old.
Pandora’s cluster- is a giant galaxy cluster resulting from the simultaneous pile-up of at least four separate, smaller galaxy clusters that took place over a span of 350 million years. The galaxies in the cluster make up less than five percent of its mass. The gas (around 20 percent) is so hot that it shines only in X-rays. Dark matter makes up around 75 percent of the cluster’s mass.
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. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics.
Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.