Colliding galaxies offer insight into dark matter

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Dark Matter is one of those mysteries of space that has largely remained unanswered for decades and continues to be illusive owing to the fact that it can neither be observed nor traced directly.

However, scientists have been using advanced techniques to look for plausible hints that help unravel the dark matter mystery and in their latest effort, they have used observations from NASA’s Hubble Space Telescope and Chandra X-ray Observatory to conclude that dark matter does not slow down when colliding with itself, meaning it interacts with itself less than previously thought.

This characteristic of dark matter, researchers say, will enable them to narrow down the options for what this mysterious substance might be.

Because of dark matter’s property of not reflecting, absorbing or emitting light, it can only be traced indirectly. This can be done by measuring how it warps space through gravitational lensing, during which the light from a distant source is magnified and distorted by the gravity of dark matter. Researchers have been studying dark matter in ways similar to visible matter – by watching what happens when it bumps into other objects. In this case, the colliding objects under observation are galaxy clusters.

Observing galactic collisions through Hubble and Chandra telescopes, scientists have come closer to garnering an understanding of the cosmic phenomenon.

Researchers used Hubble to map the distribution of stars and dark matter after a collision, which was traced through its gravitational lensing effect on background light and Chandra to detect the X-ray emission from colliding gas clouds.

“Dark matter is an enigma we have long sought to unravel,” said John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate in Washington. “With the combined capabilities of these great observatories, both in extended mission, we are ever closer to understanding this cosmic phenomenon.”

Scientists explain that in galaxy clusters, which are made primarily of galaxies, gas clouds, and dark matter, the gas clouds surrounding galaxies crash into each other and slow down or stop. The galaxies are much less affected by the drag from the gas and, because of the huge gaps between the stars within them, do not slow each other down.

“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,” said the study’s lead author David Harvey of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.

Harvey and his team studied 72 large cluster collisions. The collisions happened at different times and were viewed from different angles — some from the side, and others head-on. [Read: CERN’s upgraded LHC to restart this month; ready to look for dark matter]

The team found that, like the galaxies, the dark matter continued straight through the violent collisions without slowing down much. This means dark matter does not interact with visible particles and flies by other dark matter with much less interaction than previously thought. Had the dark matter dragged against other dark matter, the distribution of galaxies would have shifted. [Read: Gamma ray emissions from newly found dwarf galaxies hint at dark matter]

“A previous study had seen similar behavior in the Bullet Cluster,” said team member Richard Massey of Durham University in the United Kingdom. “But it’s difficult to interpret what you’re seeing if you have just one example. Each collision takes hundreds of millions of years, so in a human lifetime we only get to see one freeze-frame from a single camera angle. Now that we have studied so many more collisions, we can start to piece together the full movie and better understand what is going on.”

With this discovery, the team has successfully narrowed down the properties of dark matter. Particle physics theorists now have a smaller set of unknowns to work around when building their models.

“It is unclear how much we expect dark matter to interact with itself because dark matter already is going against everything we know,” said Harvey. “We know from previous observations that it must interact with itself reasonably weakly.”

Dark matter may have rich and complex properties, and there are still several other types of interactions to study. These latest results rule out interactions that create a strong frictional force, causing dark matter to slow down during collisions. [Read: Dark matter may have caused mass extinction, scientist says]

The team also will study other possible interactions, such as dark matter particles bouncing off each other like billiard balls and causing dark matter particles to be ejected from the clouds by collisions or for dark matter blobs to change shape. The team also is 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,” said Harvey. “These astronomically large particle colliders are finally letting us glimpse the dark world all around us, but just out of reach.”

The results are published in the March 27 edition of the journal Science.