DARK MATTER CORE DEFIES EXPLANATION IN NASA HUBBLE IMAGE

The following excerpt is from the NASA website:

"WASHINGTON -- Astronomers using data from NASA's Hubble Telescope have
observed what appears to be a clump of dark matter left behind from a
wreck between massive clusters of galaxies. The result could
challenge current theories about dark matter that predict galaxies
should be anchored to the invisible substance even during the shock
of a collision.

Abell 520 is a gigantic merger of galaxy clusters located 2.4 billion
light-years away. Dark matter is not visible, although its presence
and distribution is found indirectly through its effects. Dark matter
can act like a magnifying glass, bending and distorting light from
galaxies and clusters behind it. Astronomers can use this effect,
called gravitational lensing, to infer the presence of dark matter in
massive galaxy clusters.

This technique revealed the dark matter in Abell 520 had collected
into a "dark core," containing far fewer galaxies than would be
expected if the dark matter and galaxies were anchored together. Most
of the galaxies apparently have sailed far away from the collision.
"This result is a puzzle," said astronomer James Jee of the University
of California in Davis, lead author of paper about the results
available online in The Astrophysical Journal. "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."

Initial detections of dark matter in the cluster, made in 2007, were
so unusual that astronomers shrugged them off as unreal, because of
poor data. New results from NASA's Hubble Space Telescope confirm
that dark matter and galaxies separated in Abell 520.

One way to study 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, like a dog on a leash. Clouds of hot,
X-ray emitting intergalactic gas, however, plow into one another,
slow down, and lag behind the impact.

That theory was supported by visible-light and X-ray observations of a
colossal collision between two galaxy clusters called the Bullet
Cluster. The galactic grouping has become an example of how dark
matter should behave.

Studies of Abell 520 showed that dark matter's behavior may not be so
simple. Using the original observations, astronomers found 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. NASA's Chandra X-ray Observatory was used to
detect the hot gas. Astronomers used the Canada-France-Hawaii
Telescope and Subaru Telescope atop Mauna Kea to infer the location
of dark matter by measuring the gravitationally lensed light from
more distant background galaxies.

The astronomers then turned to the Hubble's Wide Field Planetary
Camera 2, which can detect subtle distortions in the images of
background galaxies and use this information to map dark matter. To
astronomers' surprise, the 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 said. "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. In one scenario, which would have
staggering implications, some dark matter may be what astronomers
call "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 is that Abell 520 has
resulted from a more complicated interaction than the Bullet Cluster
encounter. 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.

A third possibility is that the core contained many galaxies, but they
were 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, the group will try to create a computer
simulation to reconstruct the collision and see if it yields some
answers to dark matter's weird behavior.

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space
Flight Center in Greenbelt, Md., manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Md., conducts
Hubble science operations. STScI is operated by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C."