Coma galaxy clusters
The Coma galaxy cluster is a huge collection of over 1000 galaxies , which in the constellation Haar der Berenice (Latin Coma Berenices) occupy an angle of about 3 ° × 5 °. Due to its relative proximity, it played a major role in the study of the large-scale distribution of galaxies and is called Abell 1656 in the catalog of the astronomer George Ogden Abell .
The central area is 2 ° and, assuming a Hubble constant of , just under 100 megaparsecs (around 300 million light years ) from the sun. Unsöld , however, gives 130 Mpc. The diameter of the Coma galaxy cluster is about 20 million light years.
In the sky just to the south is the much closer Virgo Cluster , the distance of which is 15-20 Mpc. The assignment of individual galaxies in its background was only possible a few decades ago.
The Coma cluster is part of the Coma supercluster .
Arrangement of galaxies
The galaxies are not evenly distributed in space, but are grouped in superordinate clusters . The cosmologists assume that these structures in the early universe from a large scale foam-like mass distribution have formed under the influence of gravity.
Large clusters contain several thousand individual galaxies that orbit the center of gravity of the system on different orbits at 500 to 1000 km / s . They are distributed over a space with a diameter of about 3–5 Mpc. The total mass is between 10 14 and 10 15 solar masses . In the center there is usually a particularly large elliptical galaxy .
In contrast, field galaxies (see: galaxy clusters ), some of which can be found in the constellation Coma, are due to only small fluctuations in density in the primordial universe.
The constellation Coma has both manifestations to offer. Due to its location near the galactic North Pole, this viewing direction - perpendicular to the plane of the Milky Way - is hardly affected by the gas and dust clouds of the plane. Therefore, some foreground galaxies can be observed well at distances of 20 to 40 million light years (MLj), behind which there is an extension of the Virgo cluster (50–70 MLj), as well as the actual Coma galaxy cluster at 300–450 MLj distance. Beyond these clusters, even more distant galaxy structures can be made out with large instruments or the Hubble telescope . In total, the Palomar Sky Survey shows in the relevant 3 ° section of the constellation Coma almost 7000 galaxies (see 4th web link) up to about the limit size 21 mag , which extends far beyond the Coma cluster .
In addition to the two galaxy clusters mentioned, numerous smaller groups in other areas of the sky have been investigated in the last few decades, including the M81 and the Sculptor group . The more distant structures and the so-called super clusters are only gradually being explored with modern large and space telescopes, such as B. in the direction of the constellations Centaur, Great Bear (Ursa major), Hercules, Perseus / Pisces and Hydra (see web links). On extremely long-exposure " deep sky " images, there are ultimately a myriad of distant background galaxies, including some that appear distorted into arcs by the action of gravitational lenses .
Discovery story
Some brighter spiral galaxies of the Coma galaxy cluster can already be seen in larger amateur telescopes, e.g. B. NGC 4889 with 11.5 m . However, it has only become clear in the last few decades that it has over 1,000 galaxies.
The brightest 100 “nebulae” in the sky were cataloged between around 1760 and 1780, mainly by comet researcher Charles Messier . Among them were some nearby galaxies in the constellations Coma and Virgo (Virgo), but they are far ahead of the Coma cluster : the Black Eye galaxy Messier 64 in Coma and M49 and M87 in neighboring Virgo. In 1781, Messier noticed an accumulation of galaxies in this constellation, and the Messier catalog in 1784 already contained 16 objects of the Virgo cluster , but its true nature was only recognized much later. The Coma cluster, which was seven times further away, could not be identified at first, only the galaxies mentioned in its foreground.
William Herschel sighted a few distant nebulae from 1783 and had measured 23 galaxies of the Coma cluster by 1785. He noted that they are not distributed randomly, but rather pile up in some directions. His son John Herschel observed the area from 1827–1831, but did not find any particular cluster, because most of the galaxies were on the limit of visibility.
It was only Heinrich Ludwig d'Arrest who recognized the cluster of galaxies in the accumulation of these distant nebulae through systematic observations in the years 1861–1867. His refractor was only 11 inches in aperture, but it had a good field of view. Soon more nebulae were discovered in the Coma Cluster, 12 of them by Guillaume Bigourdan between 1885 and 1895 and 22 by Hermann Kobold . Max Wolf (1864–1932) is often wrongly named as the discoverer; however, it was from him that the first successful photograph of the heap (March 1901) and an analysis based on it (see illustration below) come from.
As early as 1933, Fritz Zwicky pointed out that the Coma cluster must contain a significant proportion of dark matter .
structure
While some of the spiral nebulae in the foreground have a magnitude of around 10 mag and can be seen in smaller amateur telescopes , the brightest galaxies of the actual Coma cluster (13–14 mag) require telescopes with an aperture of at least 20 cm. In order to determine the accumulation of distant galaxies without a more detailed analysis, you also need a good wide-angle eyepiece or a series of photos of very uniform quality. The enormous abundance of the heap was therefore recognized relatively late.
Its center is set approximately at the following coordinates:
- RA / Dec. (B1950.0) 12h 57.4 + 28 ° 15 '± 2'
- RA / Dec. ( J2000.0 ) 12h 59.8 + 27 ° 59 '± 2',
The mean redshift of its group members is 0.0219 (according to other sources 0.0232), which corresponds to a radial speed of 6600 to 7000 km / s.
Near the spatial center is NGC 4889 , a giant elliptical galaxy , and, unusually, a second one, NGC 4874 . Both are of the type of cD galaxies and are more than 300 MLj away from us, but only 1 MLj apart. Most of the other star systems in the central area are also elliptical in shape, which indicates their great age and the merging of numerous individual galaxies. In the lower-mass outer areas, however, spiral galaxies predominate, in whose spiral poor areas of persistent star formation can be found.
Like other comparable systems, the Coma cluster is penetrated by a thin intergalactic gas that heats up strongly around the central area as a result of the rapid movement of the galaxies belonging to the cluster. In this way it reaches temperatures of many millions of degrees and is noticeable through the emission of intense X-rays, which was first detected in 1971 . One particularly extensive X-ray source of the cluster is called Coma X-1 . In addition, the largest part of the mass must be so-called dark matter . It cannot be observed directly, but only determined by the effect of its gravitation .
Numerous radio sources were also located here in a field of 2x2 °: if one summarizes 13 detailed analyzes of the frequency ranges 150 kHz to 4.8 GHz , there are 298 radio sources, half of which (with radiation over 10 m Jansky ) were also examined spectrally .
Objects visible in a 50 cm telescope include:
- NGC 4860, 4864, 4867, 4869, 4871, 4873, 4874, 4875, 4876, 4883, 4886, 4889, 4894, 4898, 4906, 4908, 4927, 4929, 4931, and 4934;
- IC 3946, 3947, 3949, 3957, 3959, 3960, 3963, 3973, 3998, 4011, 4021, 4026, 4041, 4042, 4051, and PGC 44652.
Typing and coma superclusters
In the modern typing of galaxy clusters according to Rood and Sastry (C = with nucleus, B = binary, F = flat, L = linear, I = irregular) the Coma cluster is one of the B clusters : it is made up of a pair of cD Galaxies dominate, as the astronomer George Abell of the famous Mount Palomar observatory discovered in the 1950s . In its basic catalog it is called Abell 1656.
The whole heap is roughly spherical, but has smaller arms a few degrees apart. The numerous elliptical (older) galaxies are concentrated towards the middle, where the two oe cD galaxies are located (see IMAGE above and Palomar field image in the 1st web link). The mean distance between two galaxies there is only a third of the distance between the Milky Way and the Andromeda Nebula . The Coma cluster is an example of a very rich galaxy cluster. The galaxies have been in gravitational interaction with one another for billions of years and have thus adjusted their kinetic energies to one another.
At a slightly closer distance, and in a different direction than the Coma Cluster, a similar cluster was discovered, the so-called Leo Galaxy Cluster (Abell 1367). It belongs to type F (flat, i.e. with strong flattening ), is around 290 MLj away and, together with Abell 1656, forms part of the Coma supercluster . The two clusters (Coma and Leo) are connected by a “bridge” of galaxies, and possibly also towards the Milky Way.
The even larger Virgo galaxy cluster in the sky, however, is six times closer and has a rather irregular shape. It is combined with a few other structures to form the Virgo supercluster , which also includes the Milky Way and the Local Group . Many galaxy researchers interpret these two large structures as connected and speak of the Coma-Virgo supercluster or of a long filament : The Virgo spiral filament is probably part of a very long filament that runs from Virgo way back to the " Great Wall " at the distance of the Coma cluster (Hoffman et al. 1995), and it might even be connected, on the near side, with the "Coma-Sculptor cloud" that runs through, ie includes the Local Group. If so, we should not be surprised to observe a 'finger of God' - because we live in a finger of God.
See also
Web links
- Spitzer space telescope
- The Coma heap (with picture from the Palomar Sky Survey), University of Munich
- Coma Cluster Galaxy Catalog 19940313 (6724 field galaxies, English description) ( Memento from August 12, 2007 in the Internet Archive )
- Data and literature on the Coma cluster
- Spektrum.de : Amateur recordings [1]
Individual evidence
- ↑ A.Unsöld, B.Baschek: The new cosmos (§ 12.4); 7th edition, Springer-Verlag 2005.
- ^ F. Zwicky: The redshift of extragalactic nebulae . (PDF) In: Helvetica Physica Acta . 6, No. 2, February 1933, pp. 110-127. bibcode : 1933AcHph ... 6..110Z . doi : 10.5169 / seals-110267 .
- ↑ Coma Cluster . In: Lexikon der Astronomie , Herder-Verlag, Freiburg im Breisgau 1989, Volume 1, ISBN 3-451-21491-1 , p. 157.
- ^ Website of the CalTech University, USA.