Coma or Berenice’s Hair: The Gravitational Entanglement of a Thousand Galaxies

What is the Coma galaxy cluster and why is it an exceptional cosmic structure?

The Coma cluster (or Berenice's Hair) is a spherical, very dense galaxy cluster at its center, containing over 1,000 identified galaxies, located about 300 million light-years away. It is one of the largest gravitationally bound structures in the Universe. It is distinguished by two major features: it contains two giant elliptical galaxies at its center (likely the result of past mergers) and enormous hot gas arms at 100 million degrees emitting X-rays, covering at least half a million light-years. Visible mass accounts for only a fraction of the total mass, with the majority attributed to dark matter (total mass ~10¹⁵ solar masses).

Galaxy Clusters

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

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.

FAQ: Everything about the Coma galaxy cluster

What is a galaxy cluster and what is special about the Coma cluster?

A galaxy cluster is a cosmic structure containing hundreds of galaxies bound together by gravity. The Coma cluster is special because it is spherical and very dense at its center (over 1,000 identified galaxies). It contains not one but two giant elliptical galaxies near its center, likely the result of past mergers of smaller groups. Additionally, it is surrounded by enormous hot gas arms (temperature ~100 million degrees) emitting X-rays, covering at least 500,000 light-years.

What is the role of dark matter in the Coma cluster?

Observations of galaxy velocities (several hundred km/s) and intracluster gas show that visible mass (stars, hot gas) is insufficient to explain the observed gravitational forces. The visible mass of galaxies accounts for only about one-sixth of the hot gas mass. The majority of the cluster's mass is therefore attributed to dark matter. The total mass of the Coma cluster is estimated at about 10¹⁵ solar masses, most of which is invisible and interacts only through gravity.

Why is the Coma cluster surprisingly non-turbulent despite mergers?

Most theoretical models predict that mergers between galaxy groups like those in Coma should produce strong turbulence. However, observations of the Coma cluster show that its long hot gas arms have a smooth shape and are in a rather calm environment, despite numerous past mergers. Astrophysicists believe that large-scale magnetic fields are likely responsible for this low level of turbulence, "smoothing out" the motions of the intracluster plasma.