Image description: The image of the Coma galaxy cluster is very interesting because it shows thousands of galaxies, not in an empty universe but amidst gigantic bubbles of hot gas, visible thanks to the Chandra X-ray space telescope. This gas fills the space between the galaxies and represents a much greater mass than the galaxies themselves. Credits: X-ray: NASA / CXC / MPE / J. Sanders et al; optical: SDSS.
Galaxy clusters are the largest observable structures in our Universe. They consist of hundreds of galaxies, bound together by their own gravitational attractions.
Between the galaxies, there is a lot of matter, much more matter than within the galaxies themselves. This matter consists of gas of varying temperatures and dust grains. The hot gas, whose temperature can reach 100 million degrees, forms a plasma, which is a soup of matter where electrons are separated from nuclei. In this low-density soup, with 1000 particles per cubic meter, there are only charged particles, in other words, separated ions and electrons. This plasma is a strong emitter of X-rays.
Observations of the velocities of the galaxies and the intracluster gas show that the visible mass (stars, gas) is not enough to explain the observed gravitational forces. The majority of the cluster's mass is therefore attributed to dark matter.
The Coma Cluster is a galaxy cluster. It is spherical, very dense at its center, and contains more than 1000 identified galaxies. It is located 300 million light-years away, along with the Virgo Cluster in the constellation of Virgo.
These enormous arms of hot gas seen in the Coma galaxy cluster cover at least half a million light-years. This field of view gives us an insight into how the Coma Cluster has grown and continues to grow through mergers of small groups of galaxy clusters. It is currently one of the largest structures in the Universe held together by gravity. The optical data of this composite image show hundreds of galaxies belonging to the Coma Cluster.
All these assembled galaxies represent little mass compared to the entire gaseous cosmic structure. They contain only about one-sixth of the mass of the hot gas. Furthermore, in this image processed using Chandra's data, we only see the brightest X-ray emissions. In reality, the hot gas completely fills the field of view.
Researchers believe that these gigantic arms probably formed with the gas lost by the galaxy clusters in their movement. The gas was stripped away by the "winds" created by the movement of the galaxy cluster. In the enlarged image, one can see galaxies trailing behind them a cloud of hot gas (in pink).
Coma is an unusual galaxy cluster because it contains not one, but two giant elliptical galaxies near its center. These two giant elliptical galaxies are likely the result of past mergers of small groups of galaxies. Most theoretical models predict that mergers between groups like those in Coma produce strong turbulence, like on the surface of the sea churned by the passage of many ships. Observations of the Coma Cluster show that these long, smooth arms of hot gas are in a rather calm setting, even after numerous mergers. Although the amount of turbulence in a galaxy cluster is difficult to estimate, astrophysicists believe that large-scale magnetic fields are probably responsible for the low levels of turbulence present in Coma.
The dynamics of the galaxies within the Coma Cluster are dominated by the gravity of dark matter. The speed of the galaxies can reach several hundred kilometers per second, indicating a total cluster mass of about \(10^{15}\) solar masses.
These data on the Coma Cluster were obtained after more than six days of observation time. An article on the Chandra observations was published in the September 20, 2013 issue of Science. The lead author of the article is Jeremy Sanders, from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
Why are Galaxies, Unlike Stars, So Close to Each Other? 
Galaxies
of the Local Group 
The hidden galaxy, one of Euclid's first images 
The Virgo Cluster spans approximately three Full Moons 
Where did the dark matter in our Galaxy go? 
Merging galaxies and black holes 
Mirages created by gravitational lenses 
Mystery of the Big Bang, the problem of the horizon 
Cartwheel Galaxy Cosmic Event 
The first second of our history 
A small galaxy tears apart the large NGC 6745 
The mystery of gamma bursts 
The Cigar Explosion 
Shockwaves
Gould's belt, a stellar fireworks display 
Recombination
in cosmology 
Journey to the center of our galaxy 
Lyman-alpha
bubbles 
Andromeda in the ultraviolet 
The most beautiful galaxy clusters 
Tinkerbell merger of three galaxies 
A gigantic black hole 
Enigma
of coplanar galaxies 
The cannonball, proof of dark matter 
El Gordo galaxy cluster 
Einstein
ring and cross 
How to measure distances in the Universe? 
The Hubble sequence and types of galaxies 
The spiral shape of the galactic arms 
Even more stars, the Cigar galaxy 
The
Universe of X-rays 
The most beautiful galaxies 
The oldest galaxies in the universe 
Quasars
the nuclei of galaxies 
Sagittarius A black hole at the center of our Galaxy 
The MOND theory and its contradiction 
The first image of a black hole 
Central
area of the Milky Way 
Laniakea, our supercluster of galaxies