Dark matter core remains an enigma

Astronomers analyzing data from NASA’s Hubble Telescope have identified what seems to be a clump of dark matter left behind from a collision between massive clusters of galaxies.

As astronomer James Jee of the University of California in Davis notes, the discovery could challenge current theories about dark matter, such as the prevailing belief that galaxies remain anchored to the invisible substance even during the shock of a collision.

Indeed, Abell 520 is a gigantic merger of galaxy clusters located 2.4 billion light-years away. Dark matter isn’t visible, although its presence and distribution is detected indirectly via its effects. 

To be sure, dark matter often acts like a magnifying glass – bending and distorting light from galaxies and clusters behind it. Astronomers can use the effect, dubbed gravitational lensing, to infer the presence of dark matter in massive galaxy clusters.

This technique revealed the dark matter in Abell 520 had coalesced into a “dark core” containing far fewer galaxies than would be expected if the dark matter and galaxies were actually anchored together. Interestingly enough, most of the galaxies apparently have sailed far away from the collision. 

“This result is a puzzle,” Jee acknowledged. “Dark matter is not behaving as predicted, and it’s not obviously clear what is going on. It is difficult to explain this Hubble observation with the current theories of galaxy formation and dark matter.”

According to Jee, initial detections of dark matter in the cluster, made in 2007, were so unusual that astronomers initially dismissed them as “unreal” because of poor data. However, new results from NASA’s Hubble Space Telescope confirm that dark matter and galaxies separated in Abell 520.

One method of studying the overall properties of dark matter is by analyzing collisions between galaxy clusters, the largest structures in the universe. When galaxy clusters crash, astronomers expect galaxies to tag along with the dark matter. Nevertheless, clouds of hot, X-ray emitting intergalactic gas, plow into one another, slow down, and lag behind the impact.

Studies of Abell 520 further indicate that dark matter’s behavior may not as simple as originally believed. Using the original observations, astronomers determined the system’s core was rich in dark matter and hot gas, but contained no luminous galaxies, which normally would be seen in the same location as the dark matter. Subsequent Hubble observations helped confirm the 2007 findings.

“We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped,” Jee explained. “But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples.”

The team proposed numerous explanations for the findings, but each is unsettling for astronomers. For example, in one hypothesis, some dark matter may be what astronomers term “sticky.” Like two snowballs smashing together, normal matter slams together during a collision and slows down. However, dark matter blobs are thought to pass through each other during an encounter without slowing down. This scenario proposes that some dark matter interacts with itself and stays behind during an encounter.

Another possible explanation for the discrepancy? Abell 520 resulted from a more complicated interaction than the Bullet Cluster encounter. Meaning, Abell 520 may have formed from a collision between three galaxy clusters, instead of just two colliding systems in the case of the Bullet Cluster.

Yet a third possibility is that the core contained numerous galaxies, but they are too dim to be seen, even by Hubble. Those galaxies would have to have formed dramatically fewer stars than other normal galaxies. 

Armed with the Hubble data, Jee’s group will attempt to create a computer simulation to reconstruct the collision and see if it yields some answers to dark matter’s odd behavior.