خوشهی کهکشانی و تعدادی از گروههای شناختهشدهی آنها، بزرگترین اجرام جهان هستند. یک خوشهی کهکشانی از سه بخش تشکیل شدهاست؛ کهکشانهایی که دارای میلیاردها ستارهاند، گاز داغ میان کهکشانها و مادهی تاریک، مادهای با هویتی مرموز که بیشترین جرم کهکشان را تشکیل میدهد.
در بیشتر حالات محاسبات نشان میدهد که جرم یک خوشه بیشتر از مجموع جرم کهکشانهای عضو است با بررسی محاسبات نتیجه گرفته شد بین کهکشانها مادهٔ تاریک که احتمالاً گازهای داغ پراکنده، خردهسنگ، سیاهچاله یا اجرام ناشناخته تشکیلشده باشد، ساخته شدهاست.
Structure made up of a gravitationally-bound aggregation of hundreds of galaxies
Composite image of five galaxies clustered together just 600 million years after the Universe's birth
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 second largest known gravitationally bound structures in the universe after galaxy filaments and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. 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. Galaxy clusters should not be confused with star clusters, such as galactic clusters—also known as open clusters—which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
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).
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.
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 (see 1023 m for distance comparisons).
The spread of velocities for the individual galaxies is about 800–1000 km/s.
There are three main components of a galaxy cluster. They are tabulated below:
Name of the components
In optical observations, only galaxies are visible
Galaxy clusters are categorized as type I, II, or III based on morphology.
Galaxy clusters as measuring instruments
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.
Galaxy clusters are also used for their strong gravitational potential as gravitational lenses to boost the reach of their 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.