Coma or Berenice's Hair: The Cosmic Colossus

What is a galaxy cluster?

Definition and composition of clusters

Galaxy clusters are the largest observable structures in our Universe. They consist of hundreds of galaxies, linked together by their own gravitational attractions.

Between galaxies there is a great deal of matter, much more matter than in the galaxies themselves; this matter consists of more or less hot gas and dust grains. Hot gas, whose temperature can reach 100 million degrees, forms a plasma, i.e. a soup of matter in which electrons are separated from nuclei. In this low-density soup, 1000 particles per cubic meter, there are only charged particles, in other words ions and separated electrons. This plasma is a strong emitter of X-rays.

Observations of galaxy and intracluster gas velocities show that visible mass (stars, gas) is not sufficient to explain the observed gravitational forces. The majority of the cluster's mass is therefore attributed to dark matter.

Structure of the Coma cluster

A spherical and dense architecture

The Coma cluster is a cluster of galaxies. It is spherical, very dense at its center, and contains more than 1000 identified galaxies. It is located 300 million light-years away with the Virgo cluster in the constellation Virgo.

Hot gas arms on the scale of half a million light-years

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.

One of the largest structures held together by gravity

It is currently one of the largest structures in the Universe held together by gravity. Optical data from this composite image shows hundreds of galaxies belonging to the Coma cluster.

The preponderance of hot gas

All these galaxies assembled represent little mass compared to the whole gaseous cosmic structure. They contain only about one-sixth of the mass of hot gas. Moreover, in this image processed using Chandra data, we only see the brightest X-ray emissions. In reality, the hot gas completely fills the field of view.

Origin of the gas arms and role of magnetic fields

Arms formed by gas torn off during movements

Researchers believe that these gigantic arms probably formed with the gas lost by galaxy clusters as they moved. The gas was stripped away by the "winds" created by the movement of the galaxy cluster. In the enlarged image, we can see galaxies trailing behind them a cloud of hot gas (in pink).

A peculiarity: two giant elliptical galaxies at the center

Coma is an unusual galaxy cluster, as it contains not one, but two giant elliptical galaxies near its center. These two giant elliptical galaxies are probably the result of mergers in the past of small groups of galaxies.

Turbulent mergers... but surprisingly smooth gas

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. Observation of the Coma cluster shows that these long, smooth-shaped arms of hot gas sit in a rather calm setting, even after numerous mergers.

Magnetic fields as turbulence regulators

Although the amount of turbulence in a galaxy cluster is difficult to estimate, astrophysicists believe that it is large-scale magnetic fields that are probably responsible for the low turbulence present in Coma.

The dominance of dark matter in the Coma cluster

Speeds that betray an invisible mass

The dynamics of galaxies within the Coma cluster are dominated by the gravity of dark matter. The orbital speeds of galaxies can reach several hundred kilometers per second, values far too high to be explained by the visible mass of stars and intracluster gas alone. By applying the virial theorem, astrophysicists estimate that the total mass of the cluster reaches about 10¹⁵ solar masses.

A negligible visible mass compared to the invisible

However, the visible mass — galaxies and hot gas combined — represents only about 15% of this value. The remainder, i.e. nearly 85%, is therefore attributed to a still poorly understood form of matter: dark matter. It emits, absorbs or scatters no light, but manifests itself through its gravitational imprint on the movements of celestial objects.

Dark matter, architect of large structures

Numerical simulations of the formation of large cosmic structures show that clusters like Coma formed by the gravitational collapse of vast dark matter halos, which then attract and trap gas and galaxies. The spatial distribution of galaxies in the Coma cluster, highly concentrated at its center, is in perfect agreement with the predictions of cold dark matter (CDM (cold Dark Matter)) models.

A laboratory for unraveling the mysteries of the dark Universe

The Coma cluster is therefore one of the best laboratories for testing our understanding of dark matter and the laws of gravity on a large scale. Future observations, particularly in weak gravitational lensing or with instruments such as the Euclid space telescope, will make it possible to map the distribution of this invisible mass more finely and to constrain the properties of dark matter.