A Clash of
Clusters Provides Another Clue to Dark Matter
powerful collision of galaxy clusters has been captured
with NASA’s Chandra X-ray Observatory and Hubble Space Telescope.
Like its famous cousin, the so-called Bullet Cluster, this
clash of clusters provides striking evidence for dark matter
and insight into its properties. Like the Bullet Cluster,
this newly studied cluster
shows a clear separation between dark and ordinary matter.
X-ray (NASA CXC / Stanford / S.Allen); Optical/Lensing
(NASA / STScI / UC Santa Barbara / M.Bradac)
collision of galaxy clusters has been captured with NASA’s
Observatory and Hubble Space Telescope. Like its famous cousin,
Bullet Cluster, this clash of clusters provides striking evidence for dark
matter and insight into its properties.
Bullet Cluster, this newly studied cluster, officially known
as MACSJ0025.4-1222, shows a clear separation between dark
and ordinary matter. This helps answer a crucial question about
whether dark matter interacts with itself in ways other than
via gravitational forces.
is important because it independently verifies the results
found for the Bullet Cluster in 2006. The new results show
the Bullet Cluster is not an exception and that the earlier
results were not the product of some unknown error.
the original Bullet Cluster, MACSJ0025 formed after an incredibly
energetic collision between two large clusters in almost the
plane of the sky. In some ways, MACSJ0025 can be thought of
as a prequel to the Bullet Cluster. At its much larger distance
of 5.7 billion light years, astronomers are witnessing a collision
that occurred long before the Bullet Cluster’s.
images from Hubble, the team was able to infer the distribution
of the total mass (colored in blue) — dark and ordinary matter
— using a technique known as gravitational lensing. The Chandra
data enabled the astronomers to accurately map the position
of the ordinary matter, mostly in the form of hot gas, which
glows brightly in X-rays (pink).
difference between the Bullet Cluster and the new system is that
MACSJ0025 does not actually contain a "bullet". This
feature is a dense, X-ray bright core of gas that can be seen moving
through the Bullet Cluster. Nonetheless, the amount of energy involved
in this mammoth collision is nearly as extreme as that found in
the Bullet Cluster.
gas detected by Chandra in X-rays is seen as two pink
clumps in the image and contains most
of the "normal," or
baryonic, matter in the two clusters. The bullet-shaped
clump on the right is the hot gas from one cluster, which
through the hot gas from the other larger cluster during
the collision. Most of the matter in the clusters (blue)
is clearly separate from the normal matter (pink), giving
that nearly all of the matter in the clusters is dark.
The hot gas in each cluster was slowed by a drag force,
to air resistance, during the collision. In contrast,
the dark matter was not slowed by the impact because
it does not interact directly with itself or the gas
through gravity. Therefore, during the collision the
dark matter clumps from the two clusters moved ahead
hot gas, producing the separation of the dark and normal
matter seen in the image. Click here for
Photo: X-ray: NASA / CXC / CfA / M.Markevitch et al.;
Optical: NASA / STScI; Magellan / U.Arizona / D.Clowe et
Map: NASA / STScI;
ESO WFI; Magellan / U.Arizona / D.Clowe et al.
As the two
clusters that formed MACSJ0025 (each almost a whopping million
billion times the mass of the Sun) merged at speeds of millions
of miles per hour, the hot gas in each cluster collided and
slowed down, but the dark matter did not. The separation between
the material shown in pink and blue therefore provides direct
evidence for dark matter and supports the view that dark matter
particles interact with each other only very weakly or not
at all, apart from the pull of gravity.
One of the
great accomplishments of modern astronomy has been to establish
a complete inventory of the matter and energy content of the
Universe. The so-called dark matter makes up approximately
23 percent of this content, five times more than the ordinary
matter that can be detected by telescopes. The latest results
with MACSJ0025 once again confirm these findings.
team of astronomers in this study was led by Marusa Bradac
of the University of California Santa Barbara (UCSB), and Steve
Allen of the Kavli Institute for Particle Astrophysics and
Cosmology at Stanford and SLAC. Other collaborators included
Tommaso Treu (UCSB), Harald Ebeling (University of Hawaii),
Richard Massey (Royal Observatory Edinburgh), and R. Glenn
Morris, Anja von der Linden, and Douglas Applegate (KIPAC).
Their results will appear in an upcoming issue of The Astrophysical