Astronomers have managed to unravel a complicated collision between two giant galaxy clusters in which the clusters' vast dark matter clouds separated from the normal matter. Each of the clusters contains thousands of galaxies and is located billions of light years from Earth. Prof. Adi Tzitrin from Ben Gurion University also participated in the study
An artist's impression showing what happened when two giant galaxy clusters, known together as MACS J0018.5, collided: the dark matter in the galaxy clusters (blue) moved ahead of the associated clouds of hot gas, or normal matter (orange). Both dark matter and normal matter feel gravity, but only normal matter experiences additional effects such as shocks and eddies that slow it down during a collision. Credit: WM Keck Observatory / Adam Macarenko
Astronomers have been able to unravel a complicated collision between two giant galaxy clusters in which the clusters' vast dark matter clouds separated from the normal matter. Each of the clusters contains thousands of galaxies and is located billions of light years from Earth.
When the clusters collided, dark matter - invisible matter that feels gravity but does not emit light - moved ahead of normal matter. The new observations are the first to directly investigate the decoupling of the velocities of dark matter and normal matter.
Galaxy clusters are among the largest structures in the universe, and are held together by gravity. Only 15% of the mass in such clusters is ordinary matter, the same matter that makes up planets, people, and everything we see around us. Most of this ordinary matter is hot gas, and the rest is stars and planets. The remaining 85% of the mass of the cluster is dark matter.
During the collision that took place between the clusters, known as MACS J0018.5+1626, the galaxies themselves were almost unscathed because there is so much space between them. But when the huge reservoirs of gas between the galaxies (normal matter) collided, the gas became turbulent and very hot.
While all matter, including normal matter and dark matter, interact via gravity, normal matter also interacts via electromagnetism, which slows it down during a collision. Therefore, while the normal matter stopped, the dark matter stores in each cluster continued to advance.
Think of a giant collision between several trucks carrying sand, suggests Emily Silich, lead author of a new study describing the findings in The Astrophysical Journal. "Dark matter is like sand and it moves forward," she says. Silic is a doctoral student working with Jack Sayers, a research professor of physics at California and the study's lead author.
The discovery was made using data from the California Submillimeter Observatory (recently removed from its site in Maunaka, Hawaii and moved to Chile), the WM Keck Observatory in Maunaka, NASA's Chandra X-ray Observatory, NASA's Hubble Space Telescope, the Herschel Space Observatory and the European Space Agency's Planck (which NASA's associated science centers were based at California's IPAC), and the Atacama submillimeter telescope experiment in Chile. Some of the observations were made decades ago, while the full analysis using all data sets was conducted during the last two years.
Such a decoupling of dark matter and normal matter has been observed before, and especially in the Bullet cluster. In this collision, the hot gas can be clearly seen behind after the clusters have passed each other. The situation that occurred in MACS J0018.5 is similar, but the merger angle is rotated by about 90 degrees relative to that of the Bullet cluster.
In other words, one of the giant clusters in MACS J0018.5 is flying almost straight toward Earth while the other is speeding away. This angle gave the researchers a unique perspective from which, for the first time, it was possible to map the speed of dark matter and normal matter and clarify how they separate from each other during the collision of galaxy clusters.
"In the Bullet Cluster, it's like we're sitting in the stands watching a car race and can take beautiful pictures of the cars moving from left to right on the track," says Sayers. "In our case, it's more like we're on the track with a radar gun, standing in front of a car coming towards us and being able to measure its speed."
To measure the speed of the normal matter, or gas, in the cluster, the researchers used an observational method called the kinetic Sunyaev-Zel'dovich (SZ) effect. Sayers and colleagues made the first observation of the kinetic SZ effect on a single cosmic object, a galaxy cluster called MACS J0717, in 2013, using data from CSO (the first observations of the SZ effect on MACS J0018.5 were made in 2006).
The kinetic SZ effect occurs when photons from the early universe, the cosmic microwave background (CMB), are scattered by the electrons in the hot gas on their way to us on Earth. The photons undergo a shift, called the Doppler shift, due to the motions of the electrons in the gas clouds in our line of sight. By measuring the change in brightness of the CMB due to this change, researchers can determine the speed of the gas clouds in the galaxy clusters.
"The Sunyaev-Zeldovich effects were still a very new observational tool when Jack and I first pointed a new camera at CSO at galaxy clusters in 2006, and we had no idea there would be discoveries like this," says Sunil Golwala, professor of physics and Silich's Ph.D. supervisor. .
"We expect a series of new surprises when we install next-generation instruments on the telescope at its new home in Chile."
By 2019, researchers had made the kinetic SZ measurements on several galaxy clusters, which told them the speed of the gas, or normal matter. They also used the Keck Observatory to study the velocity of galaxies in the cluster, which they were told by proxy for the velocity of dark matter (because dark matter and galaxies behave similarly during collisions).
But at this stage of the research, the team understood little about the angles of the clusters. They only knew that one of them, MACS J0018.5, showed signs of something strange – the hot gas, or normal matter, was moving in the opposite direction to the dark matter.
"We had a complete puzzle with velocities in opposite directions, and at first we thought it might be a problem with our data. Even our colleagues who simulate galaxy clusters didn't know what was going on," says Sayers. "And then Emily stepped in and let everything go."
As part of her doctoral thesis, Silic tackled the mystery of MACS J0018.5. She turned to data from the Chandra X-ray Observatory to reveal the temperature and location of the gas in the clusters as well as the extent of the shocks of the gas.
"These galaxy cluster collisions are the most energetic phenomena since the Big Bang," Silic says. "Chandra measures the extreme temperatures of the gas and tells us about the age of the merger and how recently the clusters collided."
The team also worked with Adi Citrin from Ben-Gurion University of the Negev in Israel to use the Hubble data to map dark matter through gravitational lensing.
In addition, John Zohun of the Harvard-Smithsonian Center for Astrophysics helped the team simulate the collision of the clusters. These simulations were used in combination with data from the various telescopes to ultimately determine the geometry and developmental stage of the cluster meeting. The scientists found that, before the collision, the clusters were moving towards each other at a speed of about 3000 kilometers/second, equal to about 1% of the speed of light.
With a more complete picture of what was happening, the researchers were able to understand why dark matter and normal matter appeared to be moving in opposite directions. Although scientists say it's hard to imagine, the angle of the collision, combined with the fact that dark matter and normal matter have separated from each other, explain the strange velocity measurements.
In the future, the researchers hope that more studies like this will lead to new clues about the mysterious nature of dark matter.
"This study is a starting point for more detailed studies on the nature of dark matter," Silic says. "We have a new type of direct probe that shows how dark matter behaves differently from normal matter."
Sayers, who recalls collecting the data on this object from CSO nearly 20 years earlier, 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 lead to a completely new way of studying dark matter in clusters.”
A raw astronomical image does not look pretty. Take the image and add colors or other accents to it to make it accessible to the general public.
Why isn't this a photograph? It's an artist's simulation. Does it exist? It was possible to photograph everything. It's a lie. We're being worked on. We have telescopes. Photograph what you saw.
Comments
A raw astronomical image does not look pretty. Take the image and add colors or other accents to it to make it accessible to the general public.
Why isn't this a photograph? It's an artist's simulation. Does it exist? It was possible to photograph everything. It's a lie. We're being worked on. We have telescopes. Photograph what you saw.