Clusters of galaxies are dynamical systems that continuously grow due to the accretion of large and small portions of matter. Such a process should lead to a complex structure in the distribution of dark matter within the clusters, as well as to shock waves and “cold fronts” in hot gas. Very detailed X-ray images of the Coma galaxy cluster were obtained by telescopes of the Russian orbital observatory Spektr-RG, which has been operating near the L2 point for more than a year. Thanks to them, it was possible to study in detail the process of merging clusters, incredibly stormy and long.
The Coma Cluster of Galaxies (also known as Coma) is special. It is very massive – it contains thousands of galaxies, and the close one is located at a distance of less than 100 Mpc. This is the first object in which the presence of “dark matter” (hidden mass) has been established. This was done by astrophysicist Fritz Zwicky in 1933, and in the 1950s it became the first cluster to contain a diffuse radio halo.
In the late 1960s, the idea arose that “dark matter” could be hot intergalactic gas. Indeed, the hot gas in Coma was soon discovered by the first X-ray satellite, Uhuru. Moreover, it turned out that it is hot gas that makes up almost 80% of all normal “baryonic” matter, while the stars and galaxies of the Coma cluster contain no more than 20% of the cluster baryons (baryons are a family of elementary particles, which include nuclear particles protons and neutrons). However, the hot gas was not enough to explain the phenomenon of “dark matter” – the latter still had to be much more. The total mass of baryons in hot gas and in stars of a galaxy cluster does not exceed 15% of the total mass of the cluster.