Dark Matter Core Defies Explanation
It was the result no one wanted to believe. Astronomers have observed what appeared to be a clump of dark matter left behind during a bizarre wreck following a collision between massive clusters of galaxies.
The dark matter appears to have collected into a "dark core" while most of the galaxies seemed to have moved on,
sailing past the collision site. This result could present a challenge to basic theories of dark matter, which predict that
galaxies should be anchored to the invisible substance, even during the shock of a collision.
The initial detection, made in 2007, was so unusual that astronomers shrugged them off as unreal because of the signal's moderate significance.
The observational program that led to the 2007 discovery, the Canadian Cluster Comparison Project (CCCP), was conceived at University
of Victoria and the first set of observations were carried out at the Canada-France-Hawaii Telescope. For Arif Babul of the University of
Victoria, who co-led the CCCP, "the results were both intriguing and exciting but also engendered justified skepticism, the main criticism
being that the clump of dark matter was an artifact of ground based observations, though we confirmed the CFHT results using observations
from the Japanese Subaru telescope".
However, new results from NASA's Hubble Space
Telescope confirm that dark matter and galaxies parted ways in the gigantic merging
galaxy cluster called Abell 520, located 2.4 billion light-years away.
Now, astronomers are left with the challenge of trying to explain dark matter's seemingly
oddball behavior in this cluster.
"This result is a puzzle," said astronomer James Jee of the University of California, Davis,
leader of 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
A paper reporting the team's results has been accepted for publication in The Astrophysical
Journal and is available online.
First detected about 80 years ago, 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. It's like hearing the music but not seeing the band.
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
galaxies to tag along with the dark matter, like a dog on a leash. 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. The galactic grouping has become a
textbook example of how dark matter should behave.
But studies of Abell 520 showed 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
dark matter by measuring how the mysterious substance bends light from more distant
background galaxies, an effect called gravitational lensing.
The astronomers then turned Hubble's Wide Field Planetary Camera 2 to help bail them
out of this 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
Babul, 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 tell.
"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 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 in the case of the Bullet
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 clusters collide.
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.
The Canada-France-Hawaii Telescope is a joint facility of National Research Council of Canada,
Centre National de la Recherche Scientifique of France, and University of Hawaii.
Dark Matter and Galaxies Part Ways in Collision between Hefty Galaxy Clusters
The image on top
of the picture
shows the distribution of dark matter, galaxies, and hot gas in the
core of the merging galaxy cluster Abell 520, formed from a violent collision of massive
galaxy clusters. it is a composite of the four images shwon below.
colored picture shows the starlight from galaxies, derived from observations
by the Canada-France-Hawaii Telescope.
colored picture pinpoints the location of most of the mass in the cluster,
which is dominated by dark matter. Dark matter is an invisible substance that makes up most
of the universe's mass. The dark-matter map was derived from the Hubble Wide Field Planetary
Camera 2 observations, by detecting how light from distant objects is distorted by the
cluster galaxies, an effect called gravitational lensing.
-tinted picture shows regions of hot
gas, as detected by NASA's Chandra X-ray Observatory. The gas is evidence that a collision
image of the galaxies was taken with NASA's Hubble Space Telescope and
with the Canada-France-Hawaii Telescope in Hawaii.
The blend of blue and green in the center of the image reveals that a clump of dark matter
resides near most of the hot gas, where very few galaxies are found. This finding confirms
previous observations of a dark-matter core in the cluster. The result could present a
challenge to basic theories of dark matter, which predict that galaxies should be anchored
to dark matter, even during the shock of a collision.
Abell 520 resides 2.4 billion light-years away.
The ApJ Paper: A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens by M.J. Jee et al. is available here
University of Victoria: Arif Babul
CFHT: Christian Veillet