Virgo Cluster

The Virgo Cluster of Galaxies is a large galaxy clusters with at least 1300, but probably over 2000 galaxies. It is located in the constellation Virgo (Virgo ); its center is away from our Milky Way galaxy about 54 million light-years.

The cluster also forms the center of the local supercluster, which is therefore also called the Virgo Supercluster. The Local Group - those clusters of galaxies, which includes our own Milky Way and the Andromeda Galaxy - is like the Virgo cluster of galaxies of this supercluster.

Discovery

The first member of the Virgo cluster of galaxies was discovered in February 1771 by Charles Messier. The French astronomer had begun to calibrate all nebulae in the sky, in order to avoid confusion with the new comet. It is now known that it is in the misty clouds, Messier cataloged at that time, the giant elliptical galaxy Messier 49 acted. Later, Messier was wearing more bright member galaxies of the Virgohaufens that had been discovered in the years 1779-1781 in part by his friend Pierre Méchain, in his catalog. Among them was the giant galaxy Messier 87, also known as Virgo A because of their strong radio emission.

Overall, the Messier catalog contains 16 galaxies, which are now identified as members of the Virgohaufens: M49, M58, M59, M60, M61, M84, M85, M86, M87, M88, M89, M90, M91, M98, M99, and M100. The March of the year 1781 can therefore be expressed as the time of the discovery of the galaxy cluster, as Messier himself wore in his notes after the entry for M91 the following sentences a (translated by Kenneth Glyn Jones):

Messier has therefore already at that time recognized that these nebulae form a group and that it is not to star clusters. The nature of galaxies as star systems outside our own Milky Way as he could, almost 150 years before the first observation of individual stars in the Andromeda Nebula, of course, do not recognize.

However, the bright Sombrero galaxy M 104, which apparently belongs to this group by their position in the sky is not a member of Virgohaufens because it is much closer.

Expansion, distance and morphology

In the sky of galaxies extends over an area of ​​about 8 degrees in the constellations Virgo and Coma Berenices. By the Hubble Space Telescope succeeded in 1994 for the first time to resolve Cepheids in members of Virgohaufens. By means of the periods / brightness relationship of these stars so managed an accurate distance measurement of 65 million light years, making it an approximate diameter of the group can be calculated by about 9 million light years. Like all galaxies, the Virgo cluster but not to be understood as a closed system with a clearly defined border. The foothills of the heap go rather smoothly into the bulk structure of the supercluster over, and the expansion given here therefore refers only to the area of the brighter galaxies.

The pile consists of an average mix of spiral and elliptical galaxies. The spiral galaxies, however, are spread over a zigarrenfömiges ( prolates ) ellipsoid, which is about four times as long as wide and avoid the center of the cluster. This is a typical phenomenon in densely populated clusters of galaxies, since the spiral structure in the dense center of the cluster is destroyed by strong tidal forces.

The elliptical galaxies are distributed, however, symmetric and condense to the center of the cluster strongly. The central area includes the three giant elliptical galaxies M 49, M 60 and M 87 The largest of these three galaxies, M 87 is about 10 times as large as the other two and has a mass of about 6 trillion solar masses within a radius of 50 kpc.

The distribution of all known galaxies of the cluster of the central region have no clearly defined center. Rather, the three giant galaxies form the centers of sub-groups: heap A to M87 in the geometric center of the cluster is by far the largest of these groups with about 100 trillion solar masses, or about 300 times the mass of our Milky Way, clusters B to M 49 in the South forms a conspicuous sub- center and pile C to M 60 is finally a relatively small group in the east of heap A. the large subgroup pile a decomposes again conspicuous in two parts: the main group around the giant M 87 and a smaller group around M 84 and M 86, which together with some other bright galaxies have a linear structure, which is named after its discoverer Markarjan'sche chain ( the beginning of the Markarjan'schen chain with M 84, M 86 and the pair NGC 4435 and NGC 4438 is in the image to see ). In addition to the three major sub-groups form the further outward spiral galaxies with their companions physical substructures. A typical example is the spiral galaxy M100 is far north of the center.

The total mass of the cluster can use the so-called virial theorem of celestial mechanics and modern extensions of this theorem from the peculiar velocities, ie be calculated from the deviations of the individual velocities of the members towards the center of gravity. Because these peculiar velocities z.T. are very high (up to 1.600 km / s in the case of the galaxy IC 3258 ), one obtains a value for the total mass, which is about 1.2 trillion solar masses significantly higher than the value obtained by from a counting and weighting of galaxies would expect luminosity. This effect is known as one of the oldest evidence for the existence of so-called dark matter.

As mentioned galaxies form clusters no real closed subsystems. There is evidence that, for example, the cigar-shaped structure of the spiral galaxies in the Virgo cluster expands even further and possibly even in foothills up to the Coma cluster of galaxies, the center of the neighboring supercluster ranges. Filamentary streamers same structure seem to pull well into the area of the Coma- Sculptor cloud, which contains our Local Group.

Virgo infall

The enormous mass of wealth of the Virgo cluster leads to a gravitational attraction of the remaining galaxies in the local supercluster. The fact that our Local Group is part of the Virgo supercluster, means that the gravitational attraction of the Virgo cluster is superimposed on the general cosmological Hubblefluss. The cosmological redshift of the galaxies in the Virgohaufens is therefore about 1000 km / s, significantly lower than would be expected at the given distance. With a modern value for the Hubble constant H0 = 73 km / s / Mpc would be about 1400 km / s to be expected - this value is, for example, pretty much reached by about 60 million light-years almost equidistant less massive Fornax cluster of galaxies. The difference in speed between the Hubblefluss and the actual speed of the Local Group corresponds to a relative motion to the Virgo cluster and transmits the English name Virgo infall.

Superimposed on the Virgo infall motion of the Virgo cluster as the center of the local supercluster itself The Virgo cluster moves towards the Hydra- Centaurus Supercluster and the whole complex of all the neighboring superclusters is finally pulled in the direction of the Great Attractor, the center of the Norma galaxy cluster forms. The velocity vectors of all these movements add up to a total velocity of the Local Group of about 600 km / s in a direction between the Virgo cluster and Big attractor.

Galaxies in the Virgo cluster

There are about 30 galaxies in the Virgo cluster, which is brighter than the 10.5. Size class are: