Galactic clusters are some of the largest structures we can find in the universe. Their formation and maintenance are enabled by the force of gravity. Interestingly, only 15 percent of the mass of these clusters is made up of ordinary matter, like the kind that forms planets and all known celestial bodies. Within this ordinary matter, the largest share is hot gas, while the rest belongs to stars and planets. The remaining 85 percent of the mass belongs to elusive dark matter, whose nature is still a mystery for scientists worldwide.

During the spectacular collision between clusters known as MACS J0018.5+1626, the galaxies themselves remained largely intact due to the vast empty spaces between them. However, the enormous amounts of gas located between the galaxies collided, resulting in turbulent and overheated gases. While all types of matter, including ordinary and dark, interact gravitationally, ordinary matter also communicates via electromagnetism, which slows it down during such collisions. Thus, ordinary matter slowed down, while the masses of dark matter continued to pass through unimpeded.

Comparison to trucks and sand
To better understand this phenomenon, Emily Silich, the lead author of a recent study published in The Astrophysical Journal, compares the situation to a massive collision of trucks loaded with sand. "Dark matter behaves like sand and continues its path forward," explains Silich, who is a postgraduate student under the mentorship of Jack Sayers, a research professor of physics at Caltech.

This stunning discovery is based on data collected from the Caltech Submillimeter Observatory (recently relocated from Hawaii to Chile), the W.M. Keck Observatory, NASA’s Chandra X-ray Observatory, the Hubble Space Telescope, and the European Space Observatory's Herschel and Planck. It is important to note that some of these observations were made decades earlier, with final analyses conducted in recent years.

Previous observations of matter separation
The phenomenon of separating dark and ordinary matter is not new and was previously recorded in a cluster known as the Bullet Cluster. In that case, hot gas lags behind dark matter after galactic clusters pass through each other. In the MACS J0018.5+1626 cluster (further MACS J0018.5), the situation is similar, but the orientation of the collision is rotated about 90 degrees compared to the Bullet Cluster. This means that one of the massive clusters in MACS J0018.5 is rushing towards Earth, while the other is moving away. This orientation gave researchers a unique opportunity to map for the first time the speeds of dark and ordinary matter and clarify how their separation occurs during a collision.

"With the Bullet Cluster, it's like we're sitting in the stands watching a car race, recording them as they move from left to right," explains Sayers. "In our case, it's more like we're standing on a racetrack with radar, measuring the speed of cars coming toward us."

Advanced techniques for measuring gas speed
To accurately measure the speed of ordinary matter, or gas in the cluster, researchers relied on a technique known as the kinetic Sunyaev-Zel'dovich (SZ) effect. As far back as 2013, Sayers and his team first recorded the SZ effect on a single cosmic object, the galactic cluster MACS J0717, using data from the CSO. The first observations of the SZ effect for MACS J0018.5 date back to 2006.

The kinetic SZ effect occurs when photons from the early universe, or cosmic microwave background (CMB), scatter electrons in hot gas on their way to Earth. The photons then experience a shift, known as the Doppler shift, caused by the motion of electrons in gas clouds along our line of sight. By measuring changes in the brightness of the CMB caused by this shift, researchers were able to determine the speed of gas clouds within galactic clusters.

"The Sunyaev-Zeldovich effects were still a new tool for observation when Jack and I first directed the new camera on the CSO toward galactic clusters in 2006, with no idea what discoveries would follow," recalls Sunil Golwala, a professor of physics and mentor to Emily Silich for her PhD. "We look forward to new discoveries when we install next-generation instruments on the telescope at its new location in Chile."

Data analysis and interpretation
By 2019, the research team had performed SZ measurements in several galactic clusters, allowing them to calculate the speed of the gas, or ordinary matter. Using the Keck Observatory, they also determined the speed of galaxies within the cluster, enabling them to indirectly determine the speed of dark matter, as dark matter and galaxies behave similarly during collisions. However, at this stage of research, the team had a limited understanding of the cluster's orientation. They knew that MACS J0018.5 showed signs of unusual motion, with hot gas moving in the opposite direction of dark matter.

"We had a complete anomaly with speeds in opposite directions, and at first, we thought there was a problem with our data. Even colleagues simulating galactic clusters didn't know what was happening," says Sayers. "And then Emily got involved and untangled everything."

As part of her doctoral studies, Emily Silich took on the challenge of solving the MACS J0018.5 puzzle. Relying on data from the Chandra X-ray Observatory, she discovered the temperatures and locations of gas within the cluster and the extent to which the gas was shocked. "These cluster collisions are the most energetic phenomena after the Big Bang," explains Silich. "Chandra measures the extreme temperatures of the gas and tells us about the age of the collision and how recently the clusters collided." The team also collaborated with Adi Zitrin from Ben-Gurion University in Israel, using Hubble data to map dark matter through gravitational lensing.

Simulations and results
With the help of John ZuHone from the Center for Astrophysics at Harvard and Smithsonian, the team simulated the cluster collision. These simulations, combined with data from various telescopes, enabled them to determine the geometry and phase of the cluster collision evolution. Scientists found that before the collision, the clusters were moving toward each other at approximately 3000 kilometers per second, which is about one percent of the speed of light. With a more complete picture of the events, the researchers were able to understand why dark and ordinary matter appeared to be moving in opposite directions. While scientists admit that it is difficult to visualize, the orientation of the collision, combined with the fact that dark and ordinary matter have separated, explains the unusual speed measurement results.

In the future, researchers hope that further studies like this will provide new clues about the mysterious nature of dark matter. "This study is a starting point for more detailed research into the nature of dark matter," says Silich. "We have a new type of direct probe that shows how dark matter behaves differently from ordinary matter."

Sayers, who recalls collecting CSO data on this object almost 20 years ago, says: "It took us a long time to put all the pieces of the puzzle together, but now we finally know what's going on. We hope this will pave the way for an entirely new way of studying dark matter in clusters."

The study titled "ICM-SHOX. Paper I: Methodology overview and discovery of a gas–dark matter velocity decoupling in the MACS J0018.5+1626 merger," was funded by the National Science Foundation, the Wallace L. W. Sargent Graduate Fellowship at Caltech, the Chandra X-ray Center, the U.S.-Israel Binational Science Foundation, the Ministry of Science and Technology of Israel, the AtLAST (Atacama Large Aperture Submillimeter Telescope) project, and the Consejo Nacional de Humanidades Ciencias y Technologías.

Source: California Institute of Technology