Galaxy cluster

Galaxy clusters are immense gravitationally bound systems comprising hundreds to thousands of galaxies, vast reservoirs of superheated gas, and dominant quantities of dark matter. With total masses typically ranging from 10¹⁴ to 10¹⁵ solar masses, they are among the largest and most massive structures in the cosmos, second only to superclusters in gravitational binding energy. Their study provides valuable insight into the large-scale structure of the Universe, cosmological evolution, and the behaviour of matter under extreme conditions.

Basic properties

Galaxy clusters exhibit a wide range of observable and inferred characteristics, making them fundamental testbeds for astrophysical theories. They usually contain between 100 and 1,000 galaxies, each moving within the gravitational potential well of the cluster. The high spread of galaxy velocities, often between 800 and 1,000 km/s, reflects the enormous mass of these structures.
Typical cluster diameters lie between 1 and 5 megaparsecs, placing them among the most extended gravitational assemblages known. A well-studied example, located about 10 billion light-years from Earth, contains material equivalent to nearly 500 trillion solar masses and has been observed across multiple wavelengths, including X-ray, optical, and infrared bands. The multiwavelength approach highlights the complexity of cluster environments, where different physical processes emit radiation at distinct energies.
Galaxy clusters are distinguished from galaxy groups, which are smaller aggregates of galaxies. Clusters and groups together form superclusters, the largest coherent cosmic structures observed.

Composition

The mass budget of a galaxy cluster consists of three principal components:

• Galaxies contribute only a modest fraction of the total mass. Although they provide the clearest visible signature, their mass content is minor compared with other components.
• The intracluster medium (ICM) is composed of diffuse, superheated plasma with temperatures typically between 30 and 100 million degrees Celsius. This hot gas emits strongly in the X-ray band, providing one of the most important observational windows into cluster structure and dynamics.
• Dark matter makes up the majority of a cluster’s mass. Although invisible to optical observations, its presence is inferred from gravitational effects, including galaxy motions, gravitational lensing, and X-ray temperature distributions.

The interplay of these components shapes the evolution, energy balance, and appearance of galaxy clusters.

Cluster formation and evolution

Galaxy clusters form through hierarchical structure formation, where smaller structures merge over cosmic time to assemble larger ones. These processes release vast amounts of energy through shock waves, gas infall, and interactions between galaxies. As gas collapses into the deep gravitational wells of forming clusters, it collides with existing material, generating shocks that heat it to tens of millions of degrees. The resulting X-ray emissions are a major diagnostic of cluster evolution.
Cluster environments strongly influence galaxy evolution. Frequent interactions between galaxies can lead to galaxy mergers, tidal stripping, and the removal of gas from galactic discs, which can suppress star formation. Over time, cluster galaxies often become more gas-poor and may evolve into elliptical or lenticular systems.

Classification

Several schemes have been developed to categorise galaxy clusters according to their observable properties. The Bautz–Morgan classification groups clusters into Types I, II, and III based on the luminosity contrast between the brightest cluster galaxy and the remainder. Type I clusters contain a dominant central galaxy, while Type III clusters show more uniform brightness among constituent galaxies.
Other classification methods consider cluster shape, symmetry, or X-ray luminosity, helping astronomers evaluate how dynamically relaxed or disturbed a cluster may be.

Galaxy clusters as measuring instruments

Galaxy clusters serve as important cosmological laboratories due to their immense gravitational fields and well-characterised physical properties.
Gravitational redshift
The strong gravitational potential of a cluster affects the energy of escaping photons. Light emitted from the dense central region must climb out of a deeper gravitational well than light from the outskirts, leading to an observable wavelength shift known as gravitational redshift. Studies using data from thousands of clusters have confirmed that this redshift varies with distance from the cluster centre in a manner consistent with predictions from general relativity. These findings also support the ΛCDM model of cosmology, in which dark matter constitutes most of the Universe’s mass content.
Gravitational lensing
Galaxy clusters act as powerful gravitational lenses due to the curvature of spacetime around them. As light from background objects passes near a massive cluster, its path is bent, amplifying and distorting the image. This “cosmic magnifying glass” effect can reveal extremely faint or distant galaxies otherwise beyond the reach of current telescopes. Lensing operates across the electromagnetic spectrum, though X-ray lensing is more complex due to strong X-ray emission from the ICM.
One notable example is the Phoenix Cluster, whose gravitational lensing has enabled the observation of a distant dwarf galaxy undergoing early, high-energy star formation.

Notable galaxy clusters

Numerous galaxy clusters serve as key reference points in nearby and distant cosmic environments.
Prominent clusters in the local Universe include:

• Virgo Cluster
• Fornax Cluster
• Hercules Cluster
• Coma Cluster

A massive aggregation known as the Great Attractor, dominated by the Norma Cluster, exerts sufficient gravitational influence to produce deviations from the smooth expansion predicted by Hubble’s law.
In the distant, high-redshift Universe, notable clusters include:

• SPT-CL J0546-5345
• SPT-CL J2106-5844

These systems rank among the most massive clusters identified from the early epochs of cosmic history.
Recent observations have shown that clusters also host particle acceleration processes. Non-thermal diffuse radio emissions—manifesting as radio halos and radio relics—indicate the presence of relativistic particles and magnetic fields. High-resolution X-ray telescopes such as the Chandra X-ray Observatory have revealed intricate internal structures including cold fronts and shock waves.