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Galaxy cluster

Structure made up of a gravitationally-bound aggregation of hundreds of galaxies

Galaxy cluster

Structure made up of a gravitationally-bound aggregation of hundreds of galaxies

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, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galaxies, heated gas, and dark matter. They are the biggest known gravitationally bound structures in the universe. They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. Together, galaxy groups and clusters form superclusters.

Basic properties

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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. 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.

Composition

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. Dark matter makes up the majority of the mass of galaxy clusters, but cannot be detected optically.

ComponentMass fractionDescription
Galaxies1%In optical observations, only galaxies are visible
Intergalactic gas in intracluster medium9%Plasma between the galaxies at high temperature and emit x-ray radiation by thermal bremsstrahlung
Dark matter90%Most massive component but cannot be detected optically and is inferred through gravitational interactions

Cluster formation and evolution

As galaxy clusters form, massive amounts of energy are released due to shock waves, the heating of gas, and galaxy interactions. 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

There are many classification systems for galaxy clusters, based on characteristics such as shape symmetry, X-ray luminosity, and dominant galaxy type. 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.

Galaxy clusters as measuring instruments

Gravitational redshift

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.

Gravitational lensing

Galaxy clusters are also used for their strong gravitational potential as gravitational lenses to boost the reach of telescopes. 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. 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.

Notable galaxy clusters

Main article: List of galaxy groups and clusters

ClusterNotes
Virgo ClusterThe nearest massive galaxy cluster
Norma ClusterThe cluster at the heart of the Great Attractor
Bullet ClusterA cluster merger with the first observed separation between dark matter and normal matter
This lists some of the most notable clusters; for more clusters, see the list article.

References

References

  1. "Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen". ESA/Hubble Press Release.
  2. (2012). "Formation of Galaxy Clusters". [[Annual Review of Astronomy and Astrophysics]].
  3. "Galaxy cluster IDCS J1426".
  4. "Groups & Clusters of Galaxies".
  5. Sunyaev, R.A.. (February 25, 1972). "Formation of Clusters of Galaxies; Protocluster Fragmentation and Intergalactic Heating". Astron. Astrophys..
  6. "Clusters and Superclusters of Galaxies".
  7. (1970-12-01). "On the Classification of the Forms of Clusters of Galaxies". The Astrophysical Journal.
  8. (1978). "Bautz-Morgan classes and the luminosity function for clusters of galaxies". The Astrophysical Journal.
  9. "Galaxy Clusters Back Up Einstein's Theory of Relativity".
  10. (2019-01-02). "The Physics of Galaxy Cluster Outskirts". Space Science Reviews.
  11. (2013-08-01). "Outskirts of Galaxy Clusters". Space Science Reviews.
  12. (15 October 2019). "Astronomers use giant galaxy cluster as X-ray magnifying lens".
  13. (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]].
  14. Garner, Rob. (11 July 2022). "NASA's Webb Delivers Deepest Infrared Image of Universe Yet". [[NASA]].
  15. (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]].
  16. 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]].
  17. (14 July 2022). "Hubble vs. James Webb telescope images: See the difference". [[ABC News (United States).
  18. 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]].
  19. Atkinson, Nancy. (2 May 2022). "Now, We can Finally Compare Webb to Other Infrared Observatories". [[Universe Today]].
  20. (7 January 2014). "NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies". [[NASA]].
  21. (16 October 2014). "RELEASE 14-283 – NASA's Hubble Finds Extremely Distant Galaxy through Cosmic Magnifying Glass". [[NASA]].
  22. "Distant and ancient".
  23. "Strings of homeless stars".
  24. "From toddlers to babies".
  25. "Approaching the Universe's origins".
  26. "HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns".
  27. "Streaks and stripes".
  28. "Cosmic RELICS".
  29. "Cosmic archaeology".
  30. "Hubble pushed beyond limits to spot clumps of new stars in distant galaxy".
  31. (10 February 2015). "Hubble Sees A Smiling Lens". [[NASA]].
  32. "Image of the galaxy cluster SpARCS1049".
  33. "Magnifying the distant Universe". ESA/Hubble Picture of the Week.
  34. (October 18, 2023). "Seeing Triple".
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