Bullet Cluster and Dark Matter

Image Description: What do we see in this false-color composite image?
We see all the matter of the Bullet Cluster. The two Bullet galaxy clusters are in the blue area, and the two galactic gas clouds are seen in X-rays, in red. The blue-colored areas represent most of the mass of the clusters, i.e., dark matter, six times more massive than ordinary matter. The Bullet cluster is the smaller of the two clusters, the one that passes through the other (right blue area). The gigantic collision has "decoiffed" the two clusters of their gas halo, causing a shock wave visible in the tip of the small red spot. This shock wave has strongly compressed and thus heated the gases of the cluster to the point of reaching 100 million degrees. We can distinguish something like a bullet followed by its trail of gas. Credit: NASA CXC CfA.

The Bullet Cluster

The Bullet Cluster, also known as 1E 0657-56, is a galaxy cluster famous for its spectacular collision between two smaller clusters. Located about 3.4 billion light-years away, this cluster has been studied in detail by the Hubble Space Telescope and the Chandra X-ray Observatory. Its name, "Bullet," refers to the distinctive shape of the cluster resulting from the collision.

The Bullet Cluster is composed of thousands of galaxies, hot gas, and dark matter. X-ray observations by Chandra have revealed that the hot gas, which represents most of the baryonic matter (visible), is separated from the dark matter. This separation is direct evidence of the existence of dark matter, as it does not interact with ordinary matter in the same way as hot gas.

Observations of the Bullet Cluster have allowed astronomers to use the gravitational lensing effect to map the distribution of dark matter. By observing the distortions of the images of background galaxies, scientists have been able to confirm the presence and distribution of dark matter in the cluster. This technique has been crucial for understanding the dynamics of the collision and the internal structure of the cluster.

Dark Matter in the Bullet Cluster

Galaxy clusters are not just made up of galaxies; they are bathed in cold gas of low density (1000 particles/m3) and hot gas (10 to 100 million degrees). At these temperatures, the gas is fully ionized; it is a plasma visible in the X-ray domain. The gas is distributed much more diffusely; it fills the space between the galaxies and extends well beyond. The mass of the gas belonging to the galaxy is much greater than the mass of the galaxy itself. If we measure the gravitational dynamics of the Universe on a large scale, the mass of ordinary matter in the observable Universe represents only 4% of the total mass. 23% of the mass would be dark matter and 73% dark energy. This is described in the SCDM (Standard Cold Dark Matter) model. What we see when observing the light of stars, galaxies, and clusters is ordinary matter.

How Can We See Dark Matter?

All matter, ordinary matter and dark matter, is subject to the forces of the gravitational field. The Bullet Cluster or 1E 0657-56, observable in the Carina constellation, is the result of the collision of two galaxy clusters that occurred 150 million years ago. The study of this collision began in August 2006 and provided one of the strongest pieces of evidence for the existence of dark matter.

When clusters collide, ordinary matter (stars, gas, and dust) is disturbed by gravitational forces. In reality, heavy objects like stars do not collide; they pass by each other without meeting, because the space between the stars is immense. Stars are therefore not affected by the collision; they may be slightly accelerated or decelerated gravitationally but not destroyed. On the other hand, during the collision, the cold and hot gases that make up most of the baryonic mass of the galaxies will interact with each other; they will even be strongly and rapidly slowed down. They will mix more easily due to their atomic freedom and very weak bonding. This is what we see in the composite image.

This gigantic collision between the two clusters released considerable energy. It is in the X-ray domain that the observation of the collision gives us new insights into dark matter, as stars, gases, and dark matter behave differently during the collision.

The galaxies of the two galaxy clusters are observed in visible light; these are the multiple white spots. The hot gases of the two clusters are observed in X-rays; these are the red clouds. Dark matter is represented by the diffuse blue light.

In the image, there are hundreds of galaxies grouped together in clusters, but we mainly see a small cluster of galaxies in the right blue spot and a large cluster of galaxies in the left blue spot. The two gas envelopes of the two clusters are in red; the small red spot follows the small blue spot, and the large red spot follows the large blue spot. In reality, the small cluster of galaxies on the right has just passed through the large cluster on the left.

The gigantic collision has "decoiffed" the two clusters of their gas halo, causing a shock wave visible in the tip of the small red spot. This shock wave has strongly compressed and thus heated the gases of the cluster to the point of reaching 100 million degrees. In some places, the Chandra X-ray Observatory measured a gas movement speed of 4500 km/s.

The two clusters are now separated by 3.4 light-years, and the total mass calculated based on their speed and distance represents much more than the mass of the visible ordinary matter (galaxies seen in the optical domain and gas seen in the X-ray domain). These are the areas intentionally colored in blue that show the distribution of invisible dark matter in the cluster. In this titanic head-on collision, dark matter behaved like ordinary matter; it did not interact; it passed through the other dark matter without impact, while the interstellar gas was torn from the clusters. This caused the shock wave that we see in the red cloud in the shape of a gas bullet on the right. The clear separation of dark matter and gas clouds is considered direct evidence of the existence of dark matter.