(L–R) UVic astronomers Andisheh Mahdavi, Henk Hoekstra and Arif Babul with a photograph of the “Cosmic Train Wreck”
A recent study by a team of University of Victoria researchers has raised new questions in the international astrophysics community about the fundamental nature of dark matter.
The study, led by Dr. Andisheh Mahdavi, a postdoctoral fellow in UVic’s Department of Physics and Astronomy, detailed new observations of a massive “cosmic train wreck” between giant galaxy clusters three billion light years from Earth in a system known as Abell 520.
Each galaxy cluster in the collision contains up to 1,000 galaxies, and each galaxy has billions of stars like our sun. Cluster collisions provide an ideal opportunity for researchers to study the nature of matter and gain a better understanding of how the universe is evolving.
Using NASA’s space-based Chandra X-ray Observatory as well as the Canada-France-Hawaii optical telescope, Mahdavi and his colleagues were surprised to discover a chaotic scene in the debris left over by the merging clusters.
“Whatever happened did something really unusual to the galaxies,” he says. “It moved them all to the outer edge of the cluster core, leaving only gas and dark matter at the centre. That’s never been seen before and it’s really hard to explain.”
There are three main components to galaxy clusters: individual galaxies; hot gas in between the galaxies; and dark matter—a mysterious substance that can’t be seen but which we know exists because of its gravitational effects on visible objects, such as galaxies, stars, gas and planets.
Only a very small percentage of mass in the universe can be explained by visible matter. In fact, it’s believed that the universe contains six times more dark matter than visible matter.
Current theory predicts that dark matter and galaxies should stay together even during violent collisions. But Abell 520 is defying that theory, says Mahdavi.
“These two huge clusters of galaxies are passing through each other, and you’d expect to end up with two clumps of galaxies and dark matter with some gas in the middle.”
Instead, there is a core of dark matter and hot intergalactic gas, but no bright galaxies. And to add to the puzzle, there’s a nearby “light region”—a galaxy cluster with little or no dark matter.
The study was published in the Astrophysical Journal. In addition to Mahdavi, the study team included Drs. Hendrik Hoekstra and David Balam (both from UVic) and Dr. Peter Capak (California Institute of Technology).
“It blew us away that it looks like the galaxies are removed from the densest core of dark matter,” says Hoekstra. “This would be the first time we’ve seen such a thing and could be a huge test of our knowledge of how dark matter behaves.”
“These kinds of studies offer the best hope of understanding the elusive nature of dark matter,” says Babul. “If our results hold up to more detailed scrutiny, they’ll have a profound impact on our understanding of matter itself.”
In the coming months, the team will be using data from the Hubble Space Telescope and the Chandra X-ray Observatory to confirm their findings.
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