Galactic pile-up leaves behind mysterious dark matter core

Astronomers have found an enormous and strange clump of dark matter left behind following a violent collision of galaxy clusters.

The clump is located in the Abell 520 cluster, a diffuse collection of galaxies located 2.4 billion light-years away in the constellation Orion. The celestial object, sometimes called the Train Wreck cluster, is thought to be the remnant of a chaotic crash between several galaxy clusters.

Galaxy clusters are massive collections containing tens or even thousands of galaxies gravitationally bound together. They contain large amounts of dark matter — a strange form of matter that interacts through gravity but gives off no light — which is thought to provide an anchor attracting visible matter to a specific spot.

A 2007 study of Abell 520 showed that it was mostly typical: Wherever astronomers saw visible matter, they found a large clump of dark matter. But there was one gigantic and perplexing “dark core” that should have attracted large amounts of visible matter yet contained almost no galaxies.

Abell stood in contrast to the Bullet Cluster, a similar galaxy cluster collision, which showed a textbook example of matter and dark matter distributions. Because of this, most astronomers dismissed the dark core observation in Abell 520 as a contamination or artifact in the data.

Now, a follow up analysis with the Hubble space telescope confirms the odd dark core exists, throwing researchers for a loop. The findings appear March 2 in The Astrophysical Journal.

Though dark matter is invisible, the new study used a technique called gravitational lensing to show its presence. Massive objects distort light coming from behind them. By looking for light from a distant object and observing the amount to which it is bent, astronomers can infer the amount of mass in a particular location.

One possible explanation for the presence of this dark core would be that dark matter interacts much more strongly with itself than was previously believed. This would allow the dark matter to slam together, slow down, and stay in one place. But this goes against many other observations, which suggest that clumps of dark matter pass through each other with very little interaction.

Another possibility could be that the dark core actually contains a large number of galaxies but they are simply to faint to be seen by Hubble. More powerful telescopes would be needed in order to confirm this idea.

Finally, the collision in Abell 520 could be much more complex than astronomers currently imagine. If three or four gigantic galaxy clusters were colliding, their complex interactions could plausibly leave behind a lone clump of dark matter. Astronomers are now building computer simulations to see if this could account for the dark matter’s behavior.