A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity,<ref name="Hubble protocluster" /> with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galaxies, heated gas, and dark matter.<ref name=":0" /> They are the second-largest known gravitationally bound structures in the universe after superclusters. They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered.<ref name="Kravtsov2012">Template:Cite journal</ref> Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. Together, galaxy groups and clusters form superclusters.
Basic propertiesEdit
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.<ref name=":0">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> Details are described in the "Composition" section.
- They have total masses of 1014 to 1015 solar masses.
- They typically have diameters from 1 to 5 Mpc (see 1023 m for distance comparisons).
- The spread of velocities for the individual galaxies is about 800–1000 km/s.
CompositionEdit
Galaxy clusters have three main components. Galaxies themselves only make up a small fraction of clusters, although they are the only component we can detect in the visible spectrum. The heated gas of the intracluster medium (ICM) has a peak temperature between 30 and 100 million degrees Celsius.<ref name=":0" /> Dark matter makes up the majority of the mass of galaxy clusters, but cannot be detected optically.<ref name="Kravtsov2012" />
| Component | Mass fraction | Description |
|---|---|---|
| Galaxies | 1% | In optical observations, only galaxies are visible |
| Intergalactic gas in intracluster medium | 9% | Plasma between the galaxies at high temperature and emit x-ray radiation by thermal bremsstrahlung |
| Dark matter | 90% | Most massive component but cannot be detected optically and is inferred through gravitational interactions |
Cluster formation and evolutionEdit
As galaxy clusters form, massive amounts of energy are released due to shock waves, the heating of gas, and galaxy interactions.<ref name="Kravtsov2012" /> Gas collides with existing material which generates shock waves, heating it to tens of millions of degrees and producing X-ray emissions. Galaxy evolution within the cluster is governed by interactions between galaxies, such as galaxy mergers, and gas stripping.
ClassificationEdit
There are many classification systems for galaxy clusters, based on characteristics such as shape symmetry, X-ray luminosity, and dominant galaxy type.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Bautz-Morgan classification sorts clusters into types I, II, and III based on the relative brightness of their galaxies–type I with greatest contrast and type III with the least.<ref>Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Galaxy clusters as measuring instrumentsEdit
Gravitational redshiftEdit
Galaxy clusters have been used by Radek Wojtak from the Niels Bohr Institute at the University of Copenhagen to test predictions of general relativity: energy loss from light escaping a gravitational field. Photons emitted from the center of a galaxy cluster should lose more energy than photons coming from the edge of the cluster because gravity is stronger in the center. Light emitted from the center of a cluster has a longer wavelength than light coming from the edge. This effect is known as gravitational redshift. Using the data collected from 8000 galaxy clusters, Wojtak was able to study the properties of gravitational redshift for the distribution of galaxies in clusters. He found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster as predicted by general relativity. The result also strongly supports the Lambda-Cold Dark Matter model of the Universe, according to which most of the cosmos is made up of Dark Matter that does not interact with matter.<ref>Template:Cite magazine</ref>
Gravitational lensingEdit
Galaxy clusters are also used for their strong gravitational potential as gravitational lenses to boost the reach of telescopes.<ref>Template:Cite journal</ref> The gravitational distortion of space-time occurs near massive galaxy clusters and bends the path of photons to create a cosmic magnifying glass. This can be done with photons of any wavelength from the optical to the X-ray band. The latter is more difficult, because galaxy clusters emit a lot of X-rays.<ref>Template:Cite journal</ref> However, X-ray emission may still be detected when combining X-ray data to optical data. One particular case is the use of the Phoenix galaxy cluster to observe a dwarf galaxy in its early high energy stages of star formation.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Notable galaxy clustersEdit
.png){kind=link}
{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}Notable galaxy clusters in the relatively nearby universe include the Virgo Cluster, Fornax 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. 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.
| Cluster | Notes | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Virgo Cluster | The nearest massive galaxy cluster | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Norma Cluster | The cluster at the heart of the Great Attractor | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Bullet Cluster | A cluster merger with the first observed separation between dark matter and normal matter | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Template:Small | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
GalleryEdit
Template:Multiple images Template:Wide image
ImagesEdit
- Distant and ancient SPT0615-JD.jpg
CitationClass=web }}</ref>
- Strings of homeless stars RXC J0232.2-4420.jpg
CitationClass=web }}</ref>
- From toddlers to babies RXC J0032.1+1808.jpg
CitationClass=web }}</ref>
- Approaching the Universe's origins PSZ2 G138.61-10.84.jpg
CitationClass=web }}</ref>
- HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns.jpg
CitationClass=web }}</ref>
- Streaks and stripes Abell 2537.jpg
CitationClass=web }}</ref>
- Cosmic RELICS Abell 1300.jpg
CitationClass=web }}</ref>
- Cosmic archaeology WHL J24.3324-8.477.jpg
CitationClass=web }}</ref>
- Hubble pushed beyond limits to spot clumps of new stars in distant galaxy.jpg
CitationClass=web }}</ref>
- HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg
CitationClass=web }}</ref>
- Image of the galaxy cluster SpARCS1049.jpg
CitationClass=web }}</ref>
- PIA20052-GalaxyCluster-MOO-J1142+1527-20151103.jpg
Galaxy cluster MOO J1142+1527 discovered by the MaDCoWS survey
- Heic1401a-Abell2744-20140107.jpg
Abell 2744 galaxy cluster (HST).<ref name="NASA-20140107" />
- Magnifying the distant Universe.jpg
Magnifying the distant universe through MACS J0454.1-0300.<ref>Template:Cite news</ref>
- 14-296-GalaxyClusters-PerseusVirgo-ChandraXRay-20141027.jpg
Turbulence may prevent galaxy clusters from cooling; illustrated: Perseus Cluster and Virgo Cluster (Chandra X-ray).
- Color image of galaxy cluster MCS J0416.1–2403.jpg
MACS0416.1-2403 imaged by the HST
- Light Bends from the Beyond.jpg
The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA Hubble Space Telescope
- A Menagerie of Galaxies.jpg
A menagerie of galaxies — the galaxy cluster ACO S 295
- Cosmic Lens Flare.jpg
Cosmic lens flare
- Hubble spots three images of a distant supernova.jpg
Hubble spots three images of a distant supernova
- Galaxy cluster WHL0137-08 (sunrisearc1).jpg
A massive galaxy cluster called WHL0137-08
- El Gordo (NIRCam Image) (2023-119).png
Galaxy cluster known as "El Gordo"
- Seeing Triple (potm2302a).jpeg
Observation from the James Webb Space Telescope the massive galaxy cluster RX J2129.<ref>Template:Cite news</ref>
VideosEdit
- Artist’s impression of a protocluster forming in the early Universe.ogv
Video: Formation of galaxy cluster MRC 1138-262 (artist's concept).