Cosmic Mystery Deepens

The difficulties trying to fathom and understand what we detect and observe or see billions of light years away.

Abell 520 in the Constellation of Orion some 2.4 billion light years away, where astronomers have discovered a chaotic scene unlike any witnessed before in a collision between giant galaxy clusters. The results challenge our understanding of the way clusters merge, they possibly make us even reexamine the nature of dark matter itself.

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.
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Optical telescopes can observe the starlight from the individual galaxies, and can infer the location of dark matter by its subtle light-bending effects on distant galaxies. X-ray telescopes like Chandra detect the multimillion-degree gas.

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 (Hawaii), 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. The dark matter appears to have separated from the galaxies.

In the Bullet Cluster, the hot gas is slowed down during the collision but the galaxies and dark matter appear to continue on unimpeded. In Abell 520, it appears that the galaxies were unimpeded by the collision, as expected, while a significant amount of dark matter has remained in the middle of the cluster along with the hot gas.

While the components of Abell 520 - galaxies, hot gas, and dark matter - are found in unexpected places, the overall amount of these components totals what scientists expect.

The results lead to two possible explanations: one involving how galaxy clusters interact, and the other about the nature of dark matter itself. Both of these explanations would pose uncomfortable problems for current 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 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. 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.

Credit:
X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.

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