Astronomers have unlocked the secrets of an epic collision of two massive galaxy clusters, showing that dark matter and regular matter can actually separate during these huge events.
Located billions of light-years away, these clusters are home to thousands of galaxies and offer deep insights into the complexities of our universe.
When they collided, dark matter – an invisible substance affected by gravity but not by light – moved in front of normal matter, which includes gas and stars.
What does this mean for our understanding of cosmic structures?
As we delve deeper into these phenomena, we uncover not only the mechanics of the universe, but also broader implications that could reshape our understanding of cosmic interactions and their relevance to the natural world.
Dark matter and galaxy clusters
Galaxy clusters are huge cosmic structures held together by gravity. Only 15% of their mass is normal matter, mostly hot gas, stars and planets, while the remaining 85% is dark matter.
When the clusters, collectively known as MACS J0018.5+1626, collided, the galaxies themselves remained mostly intact due to the vast spaces between them.
However, the gas between the galaxies collided, becoming turbulent and superheated. While both dark matter and normal matter are influenced by gravity, normal matter also interacts via electromagnetism, which slowed it down during the collision.
Therefore, dark matter progressed, decoupling from normal matter.
“Dark matter is like sand and flies forward,” said lead author Emily Silich, a graduate student working with Jack Sayers, a research professor of physics at Caltech and the study’s principal investigator.
MACS J0018.5 is a real treasure
A similar decoupling between dark matter and normal matter has already been observed in the Bullet cluster. However, the orientation of the MACS J0018.5 collision was different, providing a unique perspective.
“With the Bullet Cluster, it’s like sitting in a grandstand watching a car race,” Sayers said. “In our case, it’s more like being on the home straight with a radar, standing in front of a car coming toward us and we can measure its speed.”
To measure the speed of the gas in the cluster, the researchers used the Sunyaev-Zel’dovich (SZ) kinetic effect.
This effect occurs when photons from the cosmic microwave background scatter off electrons in the hot gas, causing a Doppler shift. By measuring this shift, scientists can determine the speed of gas clouds within galaxy clusters.
“The Sunyaev-Zeldovich effects were still a very new observational tool when Jack and I first turned a new camera at the CSO on galaxy clusters in 2006,” said Sunil Golwala, professor of physics and Silich’s doctoral advisor.
“We expect a series of new surprises when we install next-generation instruments on the telescope at its new home in Chile.”
Normal matter and dark matter separate into clumps
In 2019, researchers used the SZ kinetic effect to measure the speed of gas in several galaxy clusters and data from the Keck Observatory to determine the speed of galaxies, which also indicates the speed of dark matter.
However, they noticed an anomaly in MACS J0018.5: the hot gas was moving in the opposite direction to the dark matter. At first, they thought this might be a data error.
“We had this really weird thing going on with the gears going in opposite directions,” Sayers says. “And then Emily got involved and figured it all out.”
Silich used data from the Chandra X-ray Observatory to analyze the temperature and location of the gas as well as its shock levels. “These cluster collisions are the most energetic events since the Big Bang,” she said.
The team also collaborated with Adi Zitrin of Ben-Gurion University of the Negev in Israel to map dark matter using gravitational lensing and with John ZuHone of the Harvard Smithsonian Center for Astrophysics to simulate the cluster collision.
Cluster collisions and dark matter discoveries
The researchers found that the clusters, before colliding, were moving toward each other at about 3,000 kilometers per second.
The orientation of the collision and the separation of dark matter and normal matter explain the strange velocity measurements.
This research provides a new method to directly probe the behavior of dark matter, offering insight into its mysterious nature.
“This study is a starting point for more detailed studies of 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.”
In the future, more studies like this could reveal additional clues about dark matter, helping scientists understand how such a mysterious substance interacts with the universe.
The discovery was made using data from multiple observatories, including Caltech’s Submillimeter Observatory, the W.M. Keck Observatory, NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, the European Space Agency’s Herschel Space Observatory, and the Atacama Submillimeter Telescope Experiment in Chile. Some of the data was collected decades ago, and the full analysis has taken place in recent years.
The research is published in the Journal of Astrophysics.
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