— The galaxy cluster Abell 520 has a massive dark core filled with hot gas (red) and dark matter (blue) but empty of bright galaxies (yellow and orange), suggesting that when the original clusters collided, their dark matter was stripped out (Image: X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.)
Disturbing evidence has emerged from the wreckage of an intergalactic pile-up suggesting that the already mysterious substance known as dark matter may be even less well understood than astronomers thought.
The observations come from a massive galaxy cluster called Abell 520 that lies 3 billion light years away, the product of a high-speed collision between smaller galaxy clusters. Astronomers examined the wreckage using a technique called weak lensing, which relies on the fact that the gravity of any matter in the cluster bends the light of background galaxies. This distorts their images and so reveals where the cluster's matter lies.
Abell 520 turns out to hold a massive dark core, empty of bright galaxies. Some of the core is made up of hot gas, which the team detected from its emission of X-rays, but most of it has to be something else presumably the same dark matter that astronomers detect elsewhere in the universe.
Except that dark matter and galaxies usually stick together. How have they become separated here? One possibility is that the galaxies were once in the core, along with the dark matter, but then close encounters between the galaxies threw them out to the cluster's fringes. Unfortunately, the team can't get that to happen in their computer simulations, even if they tailor the initial conditions to encourage these gravitational slingshots.
A more intriguing explanation is that when the original clusters collided, their dark matter was stripped out. Astronomers expect that to happen to gas clouds in colliding clusters, but dark matter is supposed to be more slippery, barely interacting with other matter or with itself. "We expect clouds of dark matter to flow right through each other," says team member Arif Babul at the University of Victoria in Canada.
Indeed, recent observations of the Bullet Cluster, another violent collision between clusters, seem to show that dark matter does exactly that, following the galaxies and leaving only gas behind in the middle.
Could there be two types of dark matter, the conventional slippery form and another that interacts more strongly? Babul says it is possible, but dislikes the idea of invoking yet another invisible cosmic substance to explain these observations. "It would push us in an uncomfortable direction. Maybe that is the way nature is driving us, but I can imagine there would be a lot of resistance to that idea."
Nor does he have encouraging words for supporters of an alternative theory to dark matter called MOND, which relies on a modified theory of gravity. Before the team wrote up their results, they tried and failed to fit their data using MOND. "It didn't seem very promising," Babul told New Scientist.
David Spergel at Princeton University in New Jersey, US, who is not part of the team, says the big question is whether this observation rules out the standard model of cosmology, called lambda-CDM, which assumes that dark matter only interacts very weakly.
He says it is much too early to make any such claim. "A number of things need to happen next more observations and more theoretical studies, simulations of what lensing looks like for clusters in collision," Spergel told New Scientist. "You need to make sure that this is not a chance angle that somehow gives you the impression there's more dark matter there."
The team now plans to train the Hubble Space Telescope on Abell 520 to get a closer look at its puzzling dark core.
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