2024-07-29 00:02:32
On the Fast Track: Astronomers have for the first time measured how fast dark matter moves in the cosmos – and made surprising discoveries. The measurement data from two colliding galaxy clusters revealed that the invisible “dark” component significantly overtook the hot intergalactic gas. The clusters, along with their dark matter, are racing towards each other at a relative speed of about 3,000 kilometers per second – around one percent of the speed of light.
Dark matter fills nearly the entire cosmos, yet we know almost nothing about it. It is unclear not only what it is made of, but also how this invisible form of matter is distributed and whether it can interact with itself remains open. This is because dark matter can only be observed indirectly, for example through the movements of stars or galaxies. Large galaxy clusters, which consist of about 80 to 90 percent dark matter, are considered particularly suitable objects for this purpose.
“Mergers between such massive galaxy clusters are a particularly sensitive test case for cosmological models,” explains Emily Silich from the California Institute of Technology and her colleagues. From the positions of the collision partners, one can infer where and how the visible and invisible masses of these clusters interact.
Speed Measurement in a Cosmic Collision
Now, for the first time, astronomers have succeeded in measuring the speed of dark matter during such a galaxy cluster collision – and comparing it with the speed of visible matter in the form of hot glowing gases. The test subjects were the colliding galaxy clusters MACS J0018.5+1626 – two clusters made up of thousands of galaxies located several billion light years away. The crucial point is that these galaxy clusters are moving towards our line of sight, unlike most other previously observed mergers, which move across our position.
“In the Bullet Cluster and others, we are somewhat sitting in the stands of a car race and can take nice snapshots of the racing cars as they overtake each other on the finish line,” explains Silich’s colleague Jack Sayers. “In our case, we are standing right at the edge of the track and see the racing cars rushing towards us. We can therefore measure their speed like with a radar gun.”
As a “radar gun,” the astronomers used spectral observation data from telescopes operating in the X-ray, optical, and radio ranges. Objects moving away from us produce spectral shifts into the red wavelength range, while approaching objects exhibit a blue shift. Additionally, the team utilized the so-called Sunyaev-Zel’dovich effect. This occurs when the photons of cosmic background radiation scatter off the hot, ionized gas of the galaxy clusters. From this scattering, the speed of the gas can be inferred.
Dark Matter in the Fast Lane
The analyses revealed that during the collision of the two large galaxy clusters, the visible hot intergalactic gases and the dark matter do not move at the same speed – a decoupling occurs between the two. Initially, both clusters rush towards each other at about 3,000 kilometers per second. However, their components then separate: the dark matter races ahead of the hot gas masses.
In such collisions, dark matter moves faster than the hot gas in the cluster and overtakes it initially. Silich compares this scenario to the collision of two open trucks loaded with sand: when they collide, the intertwined trucks stop, while the loose sand continues to fly through the gap between them. “The dark matter is the sand and it flies ahead,” says Silich. Complementary model simulations showed that the gas takes several hundred million years to catch up again.
Electromagnetic Braking Effect
But why does this “overtaking maneuver” of dark matter occur? The reason lies in the weak interaction of dark matter with its surroundings, as the team explains: dark matter is only influenced by gravitational effects, while the hot, ionized gas masses and also normal matter are additionally subjected to electromagnetic forces. These generate additional turbulence and interactions during the cluster collision, which slow down the gas masses.
“It took us a long time to piece all the puzzle pieces together,” says Silich. “But now we finally know what is happening there. We hope that this paves the way for studying dark matter in galaxy clusters.” (The Astrophysical Journal, 2024; doi: 10.3847/1538-4357/ad3fb5)
Source: California Institute of Technology
July 29, 2024 – Nadja Podbregar