Dark matter is a mysterious and fascinating topic. It's a strange concept, and we're not entirely clear on what it actually is. One of the strongest pieces of evidence that dark matter is a particle comes from cosmic collisions. These collisions occur primarily when galaxy clusters interact, such as the famous Bullet Cluster. The gravitational lensing effect reveals how the dark matter component couples with gas and dust in the cluster, but now astronomers have discovered another galaxy cluster collision – but it's different, and it shows the collision from a new angle.
Swiss astronomer Fritz Zwicky was the first to talk about dark matter in the 1930s, when he observed the Coma Cluster. The observations found that the galaxies in the cluster were traveling faster than could be explained by visible mass alone. Zwicky proposed the existence of a type of invisible material, known as dark matter, that was gravitationally affecting the galaxies. In the 1970s, even more evidence emerged when observations of spiral galaxies found that the other regions were rotating at the same speed as the inner regions. Again, this suggested that there was some invisible matter surrounding the stars in the galaxies. Even so, dark matter has yet to be directly observed, largely due to its complete lack of interaction with normal matter.
Galaxy clusters are a phenomenon where dark matter seems to have a significant impact. The galaxies that make them up are bound together by the force of gravity. When we explore galaxy clusters and the amount of matter that seems to be present, only about 15% is normal matter. In the case of galaxies, this is mostly in the form of hot gas, but the rest will be made up of stars, planets and even people. The remaining 85% must therefore be dark matter.
Recent observations of the collision of clusters collectively known as MACS J0018.5+1626 show that the individual galaxies are largely unscathed. In galaxy clusters, the distance between galaxies is enormous, yet the gas components have become turbulent and superheated. Typically, such events would reveal themselves through gravitational and electromagnetic effects of normal matter, but dark matter simply interacts through gravity.
Caltech’s Submillimeter Observatory, Keck Observatory on Mauna Kea, Chandra X-ray Observatory, Hubble Space Telescope, Herschel Space Observatory, and Planck Observatory were part of the project that has been observing the collision of MACS J0018.5+1626. The dissociation or decoupling of dark matter and normal matter in such collisions has been seen before in the Bullet Cluster. In this event, hot gas and normal matter lagged behind the dark matter as the clusters passed through each other. MACS J0018.5+1626 is the same event, with a similar offset between normal and dark matter. However, MACS J0018.5+1626 is oriented slightly differently, offering a unique look at this type of event.
In an attempt to understand the process, a team of researchers used a method known as the kinetic-Sunyaev-Zel'dovich effect (the spectral distortion of the cosmic microwave background through inverse Compton scattering). However, this is not the first time the effect has been observed, as a team of astronomers detected it in a cluster known as MACS J0717. Since then, multiple observations of the effect have been made, allowing astronomers to measure the speed of gas and normal matter. Measuring the speed of galaxies also allowed them to deduce the speed of dark matter.
Future studies are expected to reveal even more clues about the nature of dark matter. Observations of MACS J0018.5+1626 and, previously, the Bullet Cluster have provided a good starting point, but more detailed data are required.
Fountain : Dark matter overtakes normal matter in a collision of galaxy megaclusters
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