Dark matter seen separating from normal matter after galaxy cluster collision

Astronomers have uncovered the secrets behind 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 provide deep insights into the complexities of our universe.

When they collided, the dark matter — an invisible substance affected by gravity but not light — moved ahead of the 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 just the mechanics of the universe, but also the broader implications that could reshape our comprehension of cosmic interactions and their relevance to the natural world.

Dark matter and galaxy clusters

Galaxy clusters are immense cosmic structures held together by gravity. Only 15% of their mass is normal matter, primarily hot gas, stars, and planets, while the remaining 85% is dark matter.

During the collision of the clusters, known collectively as MACS J0018.5+1626, the galaxies themselves mostly remained undamaged due to the vast spaces between them.

However, the gas between the galaxies collided, becoming turbulent and superheated. While both dark and normal matter are influenced by gravity, the normal matter also interacts via electromagnetism, which slowed it down during the collision.

Consequently, the dark matter moved ahead, decoupling from the normal matter.

“The dark matter is like the sand and flies ahead,” said lead author Emily Silich, a graduate student working with Jack Sayers, a research professor of physics at Caltech and principal investigator of the study.

MACS J0018.5 is a treasure trove

A similar decoupling of dark and normal matter was previously 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 we are sitting in a grandstand watching a car race,” Sayers said. “In our case, it’s more like we are on the straightaway with a radar gun, standing in front of a car as it comes at us and are able to obtain its speed.”

To measure the speed of the gas in the cluster, researchers employed the kinetic Sunyaev-Zel’dovich (SZ) effect.

This effect occurs when photons from the cosmic microwave background scatter off electrons in hot gas, causing a Doppler shift. By measuring this shift, scientists can determine the speed of the 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, a professor of physics and Silich’s faculty PhD advisor.

“We look forward to a slew of new surprises when we put next-generation instruments on the telescope at its new home in Chile.”

Normal and dark matter separate in clusters

By 2019, researchers had used the kinetic SZ effect to measure the gas speed in several galaxy clusters and Keck Observatory data to determine the speed of the galaxies, which also indicates the dark matter’s speed.

However, they noticed an anomaly in MACS J0018.5: the hot gas was moving in the opposite direction to the dark matter. Initially, they thought it might be a data error.

“We had this complete oddball with velocities in opposite directions,” Sayers explained. “And then Emily got involved and untangled everything.”

Silich used data from the Chandra X-ray Observatory to analyze the temperature and location of the gas and its shock levels. “These cluster collisions are the most energetic phenomena since the Big Bang,” she said.

The team also collaborated with Adi Zitrin from Ben-Gurion University of the Negev in Israel to map the dark matter using gravitational lensing and with John ZuHone from the Center for Astrophysics at Harvard Smithsonian to simulate the cluster collision.

Cluster collisions and dark matter discoveries

The researchers discovered that the clusters, before colliding, were moving toward each other at about 3000 kilometers per second.

The collision’s orientation and the separation of dark and normal matter explained the odd velocity measurements.

This research provides a new method to directly probe the behavior of dark matter, offering insights into its mysterious nature.

“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.”

In the future, more studies like this one could reveal additional clues about dark matter, helping scientists understand how such a mysterious substance interacts with the universe.

The discovery utilized data from multiple observatories, including the Caltech 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, with full analysis occurring over the past few years.

The research is published in the Astrophysical Journal.

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