An international team of astronomers led by Dr David Harvey of the Observatory of Sauverny in Switzerland and the University of Edinburgh, UK, has studied how elusive dark matter behaves when clusters of galaxies collide.
This Hubble image shows the galaxy cluster MACS J0416.1-2403; shown in blue is a map of the dark matter found within the cluster. Image credit: NASA / ESA / D. Harvey / R. Massey / HST Frontier Fields.
Dr Harvey and his colleagues used data from the NASA/ESA Hubble Space Telescope and NASA’s Chandra X-ray Observatory to study 72 cluster collisions, including both major and minor mergers. The collisions happened at different times, and are seen from different angles – some from the side, and others head-on.
“We know how gas and stars react to these cosmic crashes and where they emerge from the wreckage. Comparing how dark matter behaves can help us to narrow down what it actually is,” explained Dr Harvey, who is the lead author of the paper in the journal Science.
The astronomers found that, like the stars, the dark matter continued straight through the violent collisions without slowing down.
However, unlike in the case of the stars, this is not because the dark matter is far away from other dark matter during the collisions. The leading theory is that dark matter is spread evenly throughout the galaxy clusters so dark matter particles frequently get very close to each other.
The reason the dark matter doesn’t slow down is because not only does it not interact with visible particles, it also interacts even less with other dark matter than previously thought.
“A previous study had seen similar behavior in the Bullet Cluster,” said co-author Dr Richard Massey of Durham University, UK.
By finding that dark matter interacts with itself even less than thought, the scientists have successfully narrowed down the properties of dark matter.
Particle physics theorists have to keep looking, but they now have a smaller set of unknowns to work with when building their models.
Dark matter could potentially have rich and complex properties, and there are still several other types of interaction to study.
These latest results rule out interactions that create a strong frictional force, causing dark matter to slow down during collisions.
Other possible interactions could make dark matter particles bounce off each other like billiard balls, causing dark matter to be thrown out of collisions or for dark matter blobs to change shape.
To further increase the number of collisions that can be studied, the researchers are also looking to study collisions involving individual galaxies, which are much more common.
“There are still several viable candidates for dark matter, so the game is not over, but we are getting nearer to an answer, Dr Harvey said.
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David Harvey et al. 2015. The nongravitational interactions of dark matter in colliding galaxy clusters. Science, vol. 347, no. 6229, pp. 1462-1465; doi: 10.1126/science.1261381
