Dark Matter Mystery Deepens in Cosmic 'Train Wreck'

Dark Matter Mystery Deepens in Cosmic 'Train Wreck'
http://www.nasa.gov/mission_pages/chandra/news/07-090.html

08.16.07

Jennifer Morcone
Marshall Space Flight Center, Huntsville, Ala.
256-544-0034
Jennifer.J.Morcone@xxxxxxxx

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@xxxxxxxxxxxxxxx

News release: 07-090


Image of Abell 520 showing aftermath of a complicated collision of
galaxy clusters Astronomers have discovered a chaotic scene unlike
any witnessed before in a cosmic "train wreck" between giant galaxy
clusters. NASA\u2019s 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.\u201d 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.

These results are scheduled to appear in the October 20th issue of
The Astrophysical Journal. Other members of the research team
included David Balam (University of Victoria) and Peter Capak
(California Institute of Technology).

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for the agency's Science Mission Directorate. The
Smithsonian Astrophysical Observatory controls science and flight
operations from the Chandra X-ray Center in Cambridge, Mass. CFHT is
a joint facility of National Research Council of Canada, Centre
National de la Recherche Scientifique of France, and University of
Hawaii.

Additional information and images are available at:

http://www.nasa.gov/chandra/

and

http://chandra.harvard.edu
.

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