http://www.physorg.com/news106487537.html

Dark matter mystery deepens in cosmic 'train wreck'

An artist's illustration of the Abell 520 system shows where the bulk of the
matter (blue) is found compared to the individual galaxies (yellow) and the
hot gas (red) in the aftermath of a massive galaxy cluster collision. The
material shown in blue is dominated by dark matter. As with the Bullet
Cluster there are large separation between the regions where the galaxies
are most common (peaks 2 and 4) and where most of the hot gas lies (peak 3).
However, unlike the Bullet Cluster, a concentration of dark matter is found
(peak 3) near the bulk of the hot gas, where very few galaxies are located.
In addition, there is an area (peak 5) where there are several galaxies but
very little dark matter. These observations conflict with the general
understanding that dark matter and the galaxies should remain together,
despite a violent collision. This raises questions about the current
understanding of how dark matter behaves. Credit: CXC/M. Weiss
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 may pose problems for current theories of
dark matter behavior.

"These results challenge our understanding of the way clusters merge," said
Dr. Andisheh Mahdavi of the University of Victoria, British Columbia. "Or,
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.

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, HI, a puzzling picture emerged. A dark matter core was found, which
also contained hot gas but no bright galaxies.

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

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.

"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, 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""

In the Bullet Cluster, known as 1E 0657-56, 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.

Mahdavi and his colleagues have 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 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.

In order to confirm and fully untangle the evidence for the Abell 520 dark
matter core, the researchers have secured time for new data from Chandra
plus the Hubble Space Telescope. With the additional observations, the team
hopes to resolve the mystery surrounding this system.

Source: Chandra X-ray Center

-- 

Ken

"Buddhism elucidates why we are sentient."
"Buddhism follows thought throughout the Universe."
"Karma means that you don't get away with anything."