The two galaxy clusters, known as MACS J0018.5+1626, contain thousands of galaxies each and are located billions of light-years away from Earth. As the clusters hurtled towards each other, dark matter traveled faster than normal matter.
This artist's conceptual illustration shows what happened when two massive clusters of galaxies, collectively known as MACS J0018.5+1626, collided. The dark matter (blue) in the clusters moves ahead of the associated hot gas clouds, or regular matter (orange). Both dark matter and regular matter feel the pull of gravity, but only the regular matter experiences additional effects like shocks and turbulence that slow it down during the collision. Image courtesy of W. M. Keck Observatory/Adam Makarenko.
Galaxy cluster mergers are a rich source of information for testing the astrophysics and cosmology of galaxy clusters.
However, the coalescence of clusters produces complex projection signals that are difficult to physically interpret from individual observation probes.
“Imagine a series of sand-carrying dump trucks colliding, and the dark matter would fly forward like sand,” says astronomer Emily Silich of the California Institute of Technology and the Harvard-Smithsonian Center for Astrophysics.
This separation of dark matter and normal matter has been observed before, most famously in the Bullet Cluster.
In this collision, hot gas can be clearly seen lagging behind dark matter after the two galaxy clusters push through each other.
The situation that occurred in MACS J0018.5+1626 is similar, but the direction of the merger is rotated about 90 degrees relative to the direction of the Bullet Cluster.
In other words, one of the giant galaxy clusters in MACS J0018.5+1626 is flying almost straight towards Earth, while the other is moving away.
This orientation gave the researchers a unique perspective to map the speeds of both dark and normal matter for the first time, and unravel how they separate during galaxy cluster collisions.
“Bullet Cluster makes you feel like you're sitting in the stands watching a car race, taking beautiful snapshots of cars moving from left to right on a straight stretch of road,” said Jack Sayers, a professor at the California Institute of Technology.
“For us, it's like standing in front of an oncoming car on a straight stretch of road with a radar gun and measuring its speed.”
To measure the velocity of ordinary matter, or gas, in galaxy clusters, the astronomers used an observational technique known as the kinetic Sunyaev-Zel'dovich (SZ) effect.
In 2013, they made the first observational detection of the kinetic SZ effect on an individual cosmic object, a galaxy cluster named MACS J0717.
The kinetic SZ effect occurs when photons from the early universe, or the cosmic microwave background radiation (CMB), are scattered by electrons in hot gas on their way to Earth.
Photons undergo a shift called the Doppler shift due to the movement of electrons in the gas cloud along the line of sight.
By measuring the change in brightness of the CMB due to this shift, astronomers can determine the speed of the gas clouds within the cluster.
By 2019, the study authors had made these motional SZ measurements in several galaxy clusters to determine the velocity of the gas, or ordinary matter.
They also measured the speed of galaxies within the cluster, which gave them an indirect idea of the speed of dark matter.
However, at this stage of the study, our understanding of the cluster orientation was limited.
All they knew was that one of them, MACS J0018.5+1626, was showing signs of something strange going on: hot gas, or regular matter, moving in the opposite direction to dark matter.
“We saw a totally strange phenomenon where the velocities were in opposite directions, which initially made us think there might be a problem with the data,” Prof Sayers said.
“Even our colleagues simulating galaxy clusters had no idea what was going on.”
Scientists then used data from NASA's Chandra X-ray Observatory to determine the temperature and location of the gas in the cluster, as well as the extent to which it is being bombarded.
“These cluster collisions are the most energetic events since the Big Bang,” Šilić said.
“Chandra will measure the extreme temperatures of the gas, which will tell us the age of the merger and how recently the galaxy cluster collision took place.”
The authors found that before the collision, the clusters were moving towards each other at about 3,000 kilometers per second, roughly 1 percent of the speed of light.
With a more complete picture of what's going on, they were able to work out why dark matter and normal matter appear to be moving in opposite directions.
They say it's hard to visualize, but the direction of the collision, combined with the fact that dark matter and normal matter separated from each other, explains the strange speed measurements.
It is hoped that more studies like this one will be conducted in the future, providing new clues about the mysterious properties of dark matter.
“This work is a starting point for more detailed studies into the nature of dark matter,” Šilić said.
“We now have a new type of direct probe that shows us how dark matter behaves differently from ordinary matter.”
of Investigation result Published in Astrophysical Journal.
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Emily M. Silich others. 2024. ICM-SHOX. I. Methodology overview and discovery of gas-dark matter velocity separation in the MACS J0018.5+1626 merger. ApJ 968, 74; doi: 10.3847/1538-4357/ad3fb5
This article is a version of a press release provided by Caltech.
