Galaxy clusters are enormous structures bound together by gravity, with only 15 percent of their mass comprising normal matter, which includes the matter forming planets and stars. The majority of this normal matter is hot gas. The remaining 85 percent is dark matter. In the collision of the clusters, known as MACS J0018.5+1626, the galaxies remained largely unaffected due to the vast distances between them. However, the normal matter, consisting mainly of gas, became turbulent and heated upon impact. While both dark matter and normal matter interact through gravity, the normal matter also experiences electromagnetic forces, causing it to slow down during the collision. This slowing did not affect the dark matter, which continued to move forward.
Emily Silich, a graduate student and lead author of a new study published in The Astrophysical Journal, compares the scenario to a collision between dump trucks carrying sand. "The dark matter is like the sand and flies ahead," she explained. Silich works with Jack Sayers, a research professor of physics at Caltech and the principal investigator of the study.
The discovery was made using data from several observatories, including the Caltech Submillimeter Observatory, the W.M. Keck Observatory, NASA's Chandra X-ray Observatory, NASA's Hubble Space Telescope, the European Space Agency's Herschel Space Observatory and Planck observatory, and the Atacama Submillimeter Telescope Experiment in Chile. Some data were collected decades ago, while the full analysis took place over the past few years.
The decoupling of dark and normal matter has been observed before, notably in the Bullet Cluster. In MACS J0018.5+1626, the clusters are oriented differently, with one cluster moving nearly straight toward Earth and the other away. This unique perspective allowed researchers to map the velocities of both dark and normal matter and observe their separation during the collision.
Sayers likened the observation to standing in front of a car with a radar gun, capturing its speed as it approaches. To measure the speed of the gas in the cluster, the team used the kinetic Sunyaev-Zel'dovich (SZ) effect. This effect involves photons from the cosmic microwave background scattering off electrons in the gas, causing a Doppler shift that researchers can measure to determine the gas's speed.
By 2019, the team had measured the kinetic SZ effect in several galaxy clusters, including MACS J0018.5, and used the Keck Observatory to determine the speed of the galaxies and, by extension, the dark matter. Initially, the researchers were puzzled by the opposite velocities of normal and dark matter in MACS J0018.5. It was only after Silich analyzed the data, including X-ray observations from Chandra, that the team understood the collision's geometry and the separation of dark and normal matter.
Silich, along with Adi Zitrin from Ben-Gurion University of the Negev and John ZuHone from the Center for Astrophysics at Harvard and Smithsonian, used gravitational lensing and simulations to map the dark matter and determine the clusters' collision dynamics. The clusters moved toward each other at approximately 3000 kilometers per second before colliding. The orientation of the collision and the separation of dark and normal matter explained the observed velocities.
The researchers hope that further studies will provide new insights into the nature of dark matter. "This study is a starting point to more detailed studies into the nature of dark matter," Silich said. "We have a new type of direct probe that shows how dark matter behaves differently from normal matter."
Sayers, who began collecting data on MACS J0018.5 nearly 20 years ago, added, "It took us a long time to put all the puzzle pieces together, but now we finally know what's going on. We hope this leads to a whole new way to study dark matter in clusters."
Research Report:ICM-SHOX. Paper I: Methodology overview and discovery of a gas-dark matter velocity decoupling in the MACS J0018.5+1626 merger
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