The Dark at the Center of a Cosmic Collision
The Abell 520 galaxy cluster is an unusual structure resulting from a major merger of far off galaxies. It has been popularly nicknamed "The Train Wreck Cluster", due to its chaotic structure. Five years ago, SF State researcher Andisheh Mahdavi and his colleagues observed an unexpected dark core at the center of Abell 520, a cosmic "train wreck" of galaxy clusters. With new space-based telescope observations, they have confirmed that the core really does exist. But they are no closer to explaining why it is there. When galaxy clusters crash into each other, the bright matter of galaxies sticks together with the mysterious substance called dark matter, leaving behind hot gases. Or at least that is what astronomers have observed in similar cosmic wrecks like the Bullet Cluster. But Myungkook James Jee of the University of California, Davis, Mahdavi and their colleagues say Abell 520 has a definite -- but bewildering -- dark matter core that is completely separated from its usual bright partners.
This particular cluster presents a major problem for prevailing theories about Dark Matter as well as most alternative theories of modified gravity, because its dark matter content does not appear to behave as expected like in other clusters. As expected, the cluster's galaxies and intergalactic gas content are separated, much like the more known Bullet Cluster. It also appears to have as many galaxies and as much intergalactic gas as is expected for a cluster of this size. However, it has a large gravitational lensing core (usually thought to be the location of the dark matter) that appears to be devoid of galaxies or other normal matter. Prevailing theories of Dark Matter believe that dark matter must always stay closely attached to the galaxies. Meanwhile, the prevailing theories also believe that only the intergalactic gas is free to separate out from both the galaxies and the dark matter, during mergers. Here, the dark core appears to have little or no correlation to any of the cluster's other components.
One analysis of the motions of 293 galaxies in the cluster field has suggested that Abell 520 is "a cluster forming at the crossing of three filaments of the large scale structure. The filament aligned with the LOS (line of sight) and projected onto the center of the forming cluster might explain the apparent massive dark core shown by gravitational lensing analysis}". In other words, the dark matter core may be associated with a filament aligned with the direction of observation and not be at the center of the cluster. However, it is only a theory with no solid proof yet.
Mahdavi, an assistant professor in the Department of Physics and Astronomy said. "There is no way that you could have cold dark matter piling up like this in a region with so few galaxies."
The researchers first identified the dark core in 2007 using a technique called gravitational lensing. Even though the dark core isn't visible, astronomers can get an idea of its location and size by observing how light from galaxies behind it is distorted by the core's gravitational pull.
"We cannot see dark matter because it does not radiate. What we see is the effect of dark matter," Jee, another author, explained. "It's similar to how we cannot see wind directly, but we can tell the presence of wind by looking at the vibration of leaves on a tree."
In this case, the galaxies behind the dark core are the tree leaves. But the 2007 observations came in part from ground-based telescopes, which can detect only a few of the galaxies lurking behind Abell 520.
The researchers decided that they needed further observations from the space-based Hubble Telescope to confirm the dark core's presence. "For every ten galaxies that we were able to see from the ground, we can see 100 from space with the Hubble," they noted, "for a total of about 4000 galaxies from space versus 400 from the ground."
The 2007 study was "a result that basically everyone wished would go away," Mahdavi said, but the new observations published in The Astrophysical Journal show "without a doubt that there is a dark matter concentration in that piece of the sky."
Their results do not put the mystery to rest, however, since the researchers also note in their study that there are no plausible scenarios yet to explain the existence of the dark core. In all other known collisions, bright galaxy matter and dark matter stay together.
Why is Abell 520 so different? It may be that our understanding of how galaxies grow and collide is incomplete, Mahdavi suggests. Alternatively, a new theory of dark matter interaction could be necessary to explain the mysterious core.
Mahdavi thinks that the first scenario is more likely, and that perhaps there are "some sort of freak initial conditions that would create this amount of dark matter."
"But the only way we understand how galaxies grow up is with supercomputer simulations," he noted. The simulations--which would include recreating galaxy cluster collisions under a variety of conditions -- help to calculate how likely it would be to spot an oddball like Abell 520. "My colleagues tell me the likelihood is nil, but now we have the responsibility to go and do the hard work to check the simulations," he said.
If the simulations don't turn up anything to show that Abell 520 is possible, Mahdavi said the mystery might be best left in the hands of particle physicists to revisit their theories about the nature and interactions of dark matter.
For further information: http://www.sfsu.edu/news/2012/spring/36.html