When two enormous clusters of galaxies collided billions of years ago, their dark matter shot right out of them, leaving behind the gas and stars that made up the remains of the clusters. Understanding this process could help us figure out the nature of dark matter and its effects on the universe.
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| When galaxy clusters collided, the dark matter (blue) sailed ahead of the normal matter (orange) W.M. Keck Observatory/Adam Makarenko |
Clashes between galaxy clusters are difficult to observe. We have to catch the collisions at exactly the right time, and at the right angle with respect to Earth, to measure the velocities of the clusters’ components. For the first time, Emily Silich at the California Institute of Technology (Caltech) and her colleagues have done just that with a merger called MACS J0018.5+1626.
To take those measurements, the researchers combined observations from seven of the most powerful telescopes on Earth and in space. “We collected the same sort of data for a big sample of clusters, and all of them look exactly as expected – and this was the only weird one,” says Jack Sayers, also at Caltech.
After this cluster merger, it looked like the dark matter was heading in the opposite direction to the regular matter. But that was a trick of geometry. In fact, the researchers found that the two had simply separated, with the regular matter slowing down and the dark matter hurtling onward.
Silich compares this cosmic collision to a crash between two trucks carrying loads of sand. The trucks themselves – representing the stars and gas – smash into one another and slow down drastically, while the sand – or dark matter – keeps moving and gets thrown everywhere. This is because dark matter only interacts with other matter gravitationally, whereas regular matter slows down due to turbulence from other types of interactions.
The dark matter ends up thrown out of the clusters entirely – which provides an excellent opportunity for astronomers to observe it on its own. “When we get this separation of dark matter and gas we can start to get at the properties of dark matter, which we don’t know very much about,” says Helen Russell at the University of Nottingham in the UK.
Although astronomers have seen this phenomenon before, this particular cluster collision is special because of its orientation in the sky. It is aligned so that the collision is in our line of sight – one cluster heading straight towards us, and the other straight away from us – which allowed the researchers to measure the velocities of the various components. They then used simulations to match the data with simple models of dark matter’s behaviour.
“Because they’re putting together all these different types of observational information, they’re able to derive things about these collisions that I would have said were impossible before looking at this work,” says Lawrence Rudnick at the University of Minnesota. “We’re now going to be able to ask questions that we couldn’t ask before.”
Perhaps the biggest of those questions is the true nature of dark matter and how it interacts with both itself and other objects around it. “With high-quality observational data, you could start playing around with different models of dark matter and potentially use those objects as a probe of dark matter in that way,” says Silich.
Observations of a single collision won’t be enough to figure out exactly how dark matter works, but this research shows that it should be possible with more detailed observations of additional cosmic smash-ups.
Journal reference:
The Astrophysical Journal DOI: 10.3847/1538-4357/ad3fb5
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