Galaxy groups and galaxy clusters
Galaxies are not evenly distributed in space, but occur in clusters in structures that have formed under the influence of gravity since the beginning of the expansion of the universe and are held together by it. Galaxy groups and galaxy clusters form even larger objects, the galaxy superclusters . Galaxies that are not obviously part of a group or cluster are called field galaxies . According to today's conception, all these structures are parts of a large-scale foam-like distribution of clusters and connecting filaments , which are grouped around voids with a low galaxy density. The development of this large-scale structure from small random density fluctuations after the Big Bang and under the influence of gravity is an important topic in cosmology .
Groups of galaxies
Smaller clusters of galaxies with fewer than 50 members in a volume of up to 10 million light years (MLj) in diameter are called groups . Their mass is about 10 13 solar masses; the speeds of its members vary by about 150 km / s. The delimitation to galaxy clusters is not sharply defined.
Our Milky Way System is also in one group, the Local Group . In addition to the Milky Way, this includes the spiral galaxy M 31 in the constellation of Andromeda , M 33 in the constellation Triangle and several accompanying galaxies such as B. the Magellanic Clouds .
Galaxy clusters have a size of up to a few thousand individual galaxies, all of which move at different speeds in the cluster's common gravitational field. According to the state of research, they are the largest structures in the universe that are gravitationally bound. The mass is around 10 14 to 10 15 solar masses in an area typically 10 to 20 MLj, with velocities that spread around 500–1000 km / s (so-called pecular velocities ). The main part of the mass of the galaxy clusters is made up of dark matter (approx. 80%).
Galaxy clusters are penetrated by a thin gas with a temperature of 10 to 100 million K, which can be observed through its X-rays. This gas makes up about 15% of the mass of a galaxy cluster. The rest of the 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 nearby 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 MLj away.