Galaxy clusters
Galaxy clusters contain up to a few thousand galaxies that move at different speeds in a common gravitational field . According to the state of research, they are the largest structures in the universe that are gravitationally bound. Their mass is around 10 14 to 10 15 solar masses in an area typically 10 to 20 million ly , with speeds that vary by around 500–1000 km / s (so-called peculiar velocities ). The main part of the mass of the galaxy clusters is made up of dark matter (approx. 80%).
Clusters of galaxies are of a thin, 10 to 100 million K hot gas penetrated, which is observable by his X-rays. This gas makes up about 15% of the mass of a galaxy cluster. The rest of the visible mass (5%) is present in the stars and planets of the galaxies. In the middle of galaxy clusters there is often a huge elliptical galaxy like M 87 in the center of the galaxy cluster closest to us, the Virgo galaxy cluster , in the constellation Virgo (Virgo) . In the constellation Chemical Furnace , in the center of the Fornax Galaxy Cluster , which is only a little further away than the Virgo Cluster , is the giant elliptical galaxy NGC 1399 . It has an extended diffuse halo and is therefore the so-called cD galaxy closest to us . Another close cluster, the galaxies of which are still visible with somewhat larger amateur telescopes , is the Coma galaxy cluster in the constellation Haar der Berenike . This galaxy cluster is a typical example of a large galaxy cluster, the center of which is dominated by two giant elliptical galaxies of the cD type . This pile is about 300 million ly away.
In the dense central regions of galaxy clusters, one usually finds elliptical galaxies, while galaxies at the edge of clusters, galaxies in groups and field galaxies are mostly spiral galaxies .
Classification of galaxy clusters
Similar to the way galaxies can be classified according to the Hubble sequence scheme , galaxy clusters can also be classified according to their morphological type. The classification is less clear and the physical explanation of the morphology is usually difficult. For this reason, no classification scheme has prevailed so far, although the scheme by Rood and Sastry (RS scheme) is enjoying some popularity, probably also because of its similarity to the Hubble scheme. According to the RS scheme, a distinction is made between the following cluster types based on the distribution of the ten brightest members:
- cD clusters are dominated by a single huge cD galaxy in the center. Examples are the cluster Abell 2634 in the constellation Pegasus, which is about 400 million ly away and dominated by NGC 7720 , and Abell 2199 , which is about 430 million ly in the constellation Hercules and in the Hercules super cluster .
- B clusters (from binary ) are dominated by a pair of cD galaxies. The prime example is the famous Coma galaxy cluster (Abell 1656), another example of the 260 million ly distant galaxy cluster Abell 569 in the constellation Lynx .
- L clusters are characterized by an approximately linear arrangement of the largest galaxies in the center. An example is the Perseus galaxy cluster (Abell 426), which is about 230 million ly away in the Perseus-Pisces supercluster .
- C clusters (from English core ) are characterized by a single dense core with several large galaxies. Examples are the Hydra galaxy cluster (Abell 1060), which is about 150 million ly away and belongs to the Hydra Centaurus supercluster , and the 210 million ly away galaxy cluster Abell 262 in the constellation Andromeda, part of the Perseus-Pisces supercluster is. The Abell 1689 galaxy cluster shown above is also of the RS type C.
- F clusters are flat , i.e. H. Strongly flattened in one direction, but without strong concentration in the center. Examples are the Leo galaxy cluster (Abell 1367) in the Coma supercluster, about 290 million ly away, and the cluster IRAS 09104 + 4109, almost 6 billion ly away .
- I clusters have an irregular structure, with no clearly defined center or with multiple centers. Examples are the Virgo galaxy cluster and the Hercules galaxy cluster (Abell 2151), a spiral-rich cluster in the supercluster of the same name at a distance of about 485 million ly.
Methods for the discovery of galaxy clusters
- The next galaxy clusters can be seen as a concentration of bright galaxies in sky images. This method was refined by George Abell and his colleagues, who from the 1950s onwards in the Palomar Observatory Sky Survey identified a total of over 4000 galaxy clusters up to redshifts of 0.2 through corresponding observations in the southern sky . Distant galaxy clusters stand out only faintly from the foreground and background of other galaxies and are therefore difficult to make out.
- Since galaxy clusters contain many bright elliptical galaxies with similar reddish colors, they reveal themselves in a large-scale study of the color and brightness of galaxies by a red sequence occurring in a small area .
- X-ray astronomy space telescopes can be used to find galaxy clusters above their hot gas.
- Small changes in the cosmic background radiation when passing through the hot gas of a galaxy cluster ( Sunyaev-Zeldovich effect ) can be identified with certain radio telescopes.
Dark matter in clusters of galaxies
A precise analysis of the proper motions of the galaxies in galaxy clusters with the virial theorem shows that the entire visible matter is insufficient to ensure that the clusters are held together by gravity. This observation was first made in 1933 by Fritz Zwicky at the Coma galaxy cluster. The hot gas later discovered in the X-ray light does not have sufficient mass either. These observations gave the first indication of a previously unexplained phenomenon that is now grouped under dark matter . The current (2015) processing and analysis of X-ray astronomy data for around one hundred of the brightest galaxy clusters seems to confirm the previous assumption that there are considerably fewer galaxy clusters than would actually be expected.
Gravitational lens effect
Galaxy clusters can focus and amplify the light from astronomical objects standing far behind them by means of their gravitation ( gravitational lens ). This amplification is an important tool in order to be able to study extremely weak galaxies in the background at redshifts up to z > 6.
See also
- JKCS 041 , 10.2 billion light years away, one of the most distant galaxy clusters
- Butcher-Oemler effect , excess of blue galaxies in more distant galaxy clusters compared to closer galaxy clusters
Web links
- The oldest fully developed galaxy cluster. At: ESO.org. March 9, 2011, accessed April 25, 2018.
Individual evidence
- ↑ The Perseus Cluster of Galaxies - Astronomy Picture of the Day of October 25, 2004 (English).
- ↑ The Hercules Cluster of Galaxies - Astronomy Picture of the Day of April 27, 2005 (English).
- ↑ Analysis of current satellite X-ray data by the University of Bonn. Informationsdienst Wissenschaft eV -idw- from December 16, 2015.