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,[1] with typical masses ranging from 1014 to 1015solar masses. Clusters consist of galaxies, heated gas, and dark matter.[2] 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.[3] Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. Together, galaxy groups and clusters form superclusters.
Composite image of five galaxies clustered together just 600 million years after the Universe's birth[1]
Basic properties
edit
Galaxy cluster IDCS J1426 is located 10 billion light-years from Earth and has the mass of almost 500 trillion suns (multi-wavelength image: X-rays in blue, visible light in green, and infrared light in red).[4]
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.[2] Details are described in the "Composition" section.
They have total masses of 1014 to 1015solar 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.
Composition
edit
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.[2] Dark matter makes up the majority of the mass of galaxy clusters, but cannot be detected optically.[3]
Component
Mass fraction
Description
Galaxies
1%
In optical observations, only galaxies are visible
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 evolution
edit
As galaxy clusters form, massive amounts of energy are released due to shock waves, the heating of gas, and galaxy interactions.[3] 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.
Classification
edit
There are many classification systems for galaxy clusters, based on characteristics such as shape symmetry, X-ray luminosity, and dominant galaxy type.[5] 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.[6][7]
Galaxy clusters as measuring instruments
edit
Gravitational redshift
edit
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.[8]
Gravitational lensing
edit
Galaxy clusters are also used for their strong gravitational potential as gravitational lenses to boost the reach of telescopes.[9] 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.[10] 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.[11]
^"Galaxy cluster IDCS J1426". Retrieved 11 January 2016.
^"Clusters and Superclusters of Galaxies". ned.ipac.caltech.edu. Retrieved 2025-02-17.
^Bautz, L. P.; Morgan, W. W. (1970-12-01). "On the Classification of the Forms of Clusters of Galaxies". The Astrophysical Journal. 162: L149. doi:10.1086/180643. ISSN 0004-637X.
^Chu, Jennifer (15 October 2019). "Astronomers use giant galaxy cluster as X-ray magnifying lens". MIT News. Retrieved 2022-04-04.
^Chow, Denise; Wu, Jiachuan (12 July 2022). "Photos: How pictures from the Webb telescope compare to Hubble's - NASA's $10 billion telescope peers deeper into space than ever, revealing previously undetectable details in the cosmos". NBC News. Retrieved 16 July 2022.
^Garner, Rob (11 July 2022). "NASA's Webb Delivers Deepest Infrared Image of Universe Yet". NASA. Archived from the original on 12 July 2022. Retrieved 12 July 2022.
^Overbye, Dennis; Chang, Kenneth; Tankersley, Jim (11 July 2022). "Biden and NASA Share First Webb Space Telescope Image – From the White House on Monday, humanity got its first glimpse of what the observatory in space has been seeing: a cluster of early galaxies". The New York Times. Archived from the original on 12 July 2022. Retrieved 12 July 2022.
^Pacucci, Fabio (15 July 2022). "How Taking Pictures of 'Nothing' Changed Astronomy - Deep-field images of "empty" regions of the sky from Webb and other space telescopes are revealing more of the universe than we ever thought possible". Scientific American. Retrieved 16 July 2022.
^Deliso, Meredith; Longo, Meredith; Rothenberg, Nicolas (14 July 2022). "Hubble vs. James Webb telescope images: See the difference". ABC News. Retrieved 15 July 2022.
^Kooser, Amanda (13 July 2012). "Hubble and James Webb Space Telescope Images Compared: See the Difference - The James Webb Space Telescope builds on Hubble's legacy with stunning new views of the cosmos". CNET. Retrieved 16 July 2022.
^Atkinson, Nancy (2 May 2022). "Now, We can Finally Compare Webb to Other Infrared Observatories". Universe Today. Archived from the original on 10 May 2022. Retrieved 12 May 2022.
^ abClavin, Whitney; Jenkins, Ann; Villard, Ray (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". NASA. Retrieved 8 January 2014.
^Chou, Felecia; Weaver, Donna (16 October 2014). "RELEASE 14-283 – NASA's Hubble Finds Extremely Distant Galaxy through Cosmic Magnifying Glass". NASA. Retrieved 17 October 2014.
^"Distant and ancient". www.spacetelescope.org. Retrieved 6 May 2019.
^"Strings of homeless stars". www.spacetelescope.org. Retrieved 11 June 2018.
^"From toddlers to babies". www.spacetelescope.org. Retrieved 7 May 2018.
^"Approaching the Universe's origins". www.spacetelescope.org. Retrieved 16 April 2018.
^"HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns". www.eso.org. Retrieved 25 December 2017.
^"Streaks and stripes". www.spacetelescope.org. Retrieved 27 November 2017.
^"Cosmic RELICS". www.spacetelescope.org. Retrieved 6 November 2017.
^"Cosmic archaeology". www.spacetelescope.org. Retrieved 24 October 2017.
^"Hubble pushed beyond limits to spot clumps of new stars in distant galaxy". www.spacetelescope.org. Retrieved 12 July 2017.
^Loff, Sarah; Dunbar, Brian (10 February 2015). "Hubble Sees A Smiling Lens". NASA. Retrieved 10 February 2015.
^"Image of the galaxy cluster SpARCS1049". Retrieved 11 September 2015.
^"Magnifying the distant Universe". ESA/Hubble Picture of the Week. Retrieved 10 April 2014.