New results from
NASA's Hubble Space Telescope confirm that, contrary to
predictions, dark matter -- the invisible substance that
makes up much of our universe -- and galaxies parted ways in
the collision of two galaxy clusters 2.4 billion light-years
away. Now, astronomers are left trying to explain dark
matter's seemingly oddball behavior in the Abell 520 merging
"This result is a puzzle," said astronomer
James Jee, project scientist in the Department of Physics at
the University of California, Davis, who led the Hubble
study. "Dark matter is not behaving as predicted, and it's
not obviously clear what is going on. Theories of galaxy
formation and dark matter must explain what we are seeing."
During the collision of galaxy clusters that
formed Abell 520, the dark matter collected into a "dark
core" containing far fewer galaxies than would be expected
if the dark matter and galaxies hung together. Most of the
galaxies apparently have sailed far away from the collision.
Current theories of dark matter predict that
galaxies should be anchored to the invisible substance, even
during the shock of a collision. The initial observations,
made in 2007, were so unusual that astronomers shrugged them
off as the result of poor data.
A paper reporting the
team's results has been accepted for publication in The
Astrophysical Journal and is available online.
About dark matter
Dark matter is thought to be the
gravitational "glue" that holds galaxies together. The
mysterious invisible substance is not made of the same kind
of matter that makes up stars, planets and people.
Astronomers know little about dark matter, yet it accounts
for most of the universe's mass. They have deduced dark
matter's existence by observing its ghostly gravitational
influence on normal matter.
One way to study dark matter is by analyzing
smashups between galaxy clusters, the largest structures in
the universe. When galaxy clusters collide, astronomers
expect the galaxies to tag along with the dark matter.
Clouds of 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, and of other colliding clusters, including
one nicknamed “Perry’s cluster” which was recently described
by another team led by UC Davis astronomers.
Galaxy cluster's odd
But studies of Abell 520 have shown that dark
matter's behavior may not be so simple. The original
observations found that 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 detected the hot
gas. Astronomers used the Canada-France-Hawaii and Subaru
telescopes atop Mauna Kea to infer the location of the dark
matter by measuring how the mysterious substance bends light
from more distant background galaxies, an effect called
The astronomers then turned to Hubble's Wide
Field Planetary Camera 2 to help bail them out of their
cosmic conundrum. Instead, to their chagrin, the Hubble
observations helped confirm the earlier findings.
Astronomers used Hubble to map the dark matter in the
cluster through the gravitational lensing technique.
"Observations like those of Abell 520 are
humbling in the sense that in spite of all the leaps and
bounds in our understanding, every now and then, we are
stopped cold," explained Arif Babul of the University of
Victoria in British Columbia, the team's senior theorist.
Is Abell 520 an oddball or is the prevailing
picture of dark matter flawed? Jee thinks it's too soon to
"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
The team has proposed a half-dozen
explanations for the findings, but each is unsettling for
astronomers. "It's pick your poison," said team member
Andisheh Mahdavi of San Francisco State University in
California, who led the original Abell 520 observations in
2007. One possible explanation for the discrepancy is that
Abell 520 was 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, as in the case of the Bullet Cluster.
Another scenario is that some dark matter may
be what astronomers call "sticky." Like two snowballs
smashing together, normal matter slams into each other
during a collision and slows down. But 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 when galaxy
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 formed
dramatically fewer stars than other normal galaxies.
Armed with the Hubble data, the group hopes
to create a computer simulation to try to reconstruct the
collision, hoping that it yields some answers to dark
matter's weird behavior.
For images and more information about Abell
520's dark core, visit: http://hubblesite.org/news/2012/10 andhttp://www.nasa.gov/hubble.
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.