Dark matter mysteriously found devoid of galaxies in comic train wreck

Astronomers have discovered a chaotic scene unlike any witnessed before in a cosmic train wreck between giant galaxy clusters.

Washington, Aug 17 : Astronomers have discovered a chaotic scene unlike any witnessed before in a cosmic "train wreck" between giant galaxy clusters.

NASA's Chandra X-ray Observatory and optical telescopes revealed a dark matter core that was mostly devoid of galaxies, which astronomers believe, could pose problems for current theories of dark matter behaviour.

"These results challenge our understanding of the way clusters merge, or they possibly make us even re-examine the nature of dark matter itself," said Dr. Andisheh Mahdavi of the University of Victoria, British Columbia.

"It blew us away that it looks like the galaxies are removed from the densest core of dark matter. 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," said Dr. Hendrik Hoekstra, also of University of Victoria.

There are three main components to galaxy clusters: individual galaxies composed of billions of stars, hot gas in between the galaxies, and dark matter, a mysterious substance that dominates the cluster mass and can be detected only through its gravitational effects.

A popular theory of dark matter predicts that dark matter and galaxies should stay together, even during a violent collision, as observed in the case of the so-called Bullet Cluster.

However, when the Chandra data of the galaxy cluster system known as Abell 520 was mapped along with the optical data from the Canada-France-Hawaii Telescope and Subaru Telescope atop Mauna Kea, HI, a puzzling picture emerged.

A dark matter core was found, which also contained hot gas but no bright galaxies.

In addition to the dark matter core, a corresponding "light region" containing a group of galaxies with little or no dark matter was also detected.

Astronomers said the dark matter appeared to have separated from the galaxies.

"The observation of this group of galaxies that is almost devoid of dark matter flies in the face of our current understanding of the cosmos," said Dr. Arif Babul from the University of Victoria.

"Our standard model is that a bound group of galaxies like this should have a lot of dark matter. What does it mean that this one doesn't," he said.

The researchers say there are two possible explanations for their findings, both of which are uncomfortable for prevailing theories.

The first option is that the galaxies were separated from the dark matter through a complex set of gravitational "slingshots".

This explanation is, however, problematic because computer simulations have not been able to produce slingshots that are nearly powerful enough to cause such a separation.

The second option is that dark matter is affected not only by gravity, but also by an as-yet-unknown interaction between dark matter particles.

However, this exciting alternative would require new physics and could be difficult to reconcile with observations of other galaxies and galaxy clusters, such as the aforementioned Bullet Cluster, said Mahdavi.

The results are scheduled to appear in the October 20 issue of The Astrophysical Journal, reports Nature.

ANI