Using NASA's
Hubble Space Telescope, a team of astronomers has
discovered a ghostly ring of dark matter that formed long
ago during a titanic collision between two massive galaxy
clusters. The ring's discovery is among the strongest
evidence yet that dark matter exists.
Astronomers have long suspected the existence of the
invisible substance as the source of additional gravity
that holds together galaxy clusters. Such clusters would
fly apart if they relied only on the gravity from their
visible stars. Although astronomers don't know what dark
matter is made of, they hypothesize that it is a type of
elementary particle that pervades the universe.
"This is the first time we have detected dark matter
as having a unique structure that is different from both
the gas and galaxies in the cluster," said team member M.
James Jee of the Henry
A. Rowland Department of Physics and Astronomy at Johns
Hopkins.
The researchers spotted the ring unexpectedly while
they were mapping the distribution of dark matter within
the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located 5
billion light-years from Earth. The ring measures 2.6
million light-years across. Although astronomers cannot see
dark matter, they can infer its existence in galaxy
clusters by observing how its gravity bends the light of
more distant background galaxies.
"Although the invisible matter has been found before
in other galaxy clusters, it has never been detected to be
so largely separated from the hot gas and the galaxies that
make up galaxy clusters," Jee said. "By seeing a
dark-matter structure that is not traced by galaxies and
hot gas, we can study how it behaves differently from
normal matter."
During the scientists' dark-matter analysis, they
noticed a ripple in the mysterious substance, somewhat like
the ripples created in a pond from a stone plopping into
the water.
"I was annoyed when I saw the ring because I thought
it was an artifact, which would have implied a flaw in our
data reduction," Jee explained. "I couldn't believe my
result. But the more I tried to remove the ring, the more
it showed up. It took more than a year to convince myself
that the ring was real. I've looked at a number of
clusters, and I haven't seen anything like this."
Curious about why the ring was in the cluster and how
it had formed, Jee found previous research that suggested
the cluster had collided with another cluster 1 billion to
2 billion years ago.
The research, published in 2002 by Oliver Czoske of
the Argeleander-Institut fur Astronomie at the Universitat
Bonn, was based on spectroscopic observations of the
cluster's three-dimensional structure. The study revealed
two distinct groupings of galaxy clusters, indicating a
collision between both clusters.
Astronomers have a head-on view of the collision
because it occurred fortuitously along Earth's line of
sight. From this perspective, the dark-matter structure
looks like a ring.
Computer simulations of galaxy cluster collisions,
created by the team, show that when two clusters smash
together, the dark matter falls to the center of the
combined cluster and sloshes back out. As the dark matter
moves outward, it begins to slow down under the pull of
gravity and pile up, like cars bunched up on a freeway.
"By studying this collision, we are seeing how dark
matter responds to gravity," said team member Holland Ford,
also of Johns Hopkins. "Nature is doing an experiment for
us that we can't do in a lab, and it agrees with our
theoretical models."
Dark matter makes up most of the universe's material.
Ordinary matter, the stuff of stars and planets, comprises
only a few percent of the universe's matter.
Tracing dark matter is not an easy task because it
does not shine or reflect light.
Astronomers can only detect its influence by how its
gravity affects light. To find it, astronomers study how
faint light from more distant galaxies is distorted and
smeared into arcs and streaks by the gravity of the dark
matter in a foreground galaxy cluster, a powerful trick
called gravitational lensing. By mapping the distorted
light, astronomers can deduce the cluster's mass and trace
how dark matter is distributed in the cluster.
"The collision between the two galaxy clusters created
a ripple of dark matter that left distinct footprints in
the shapes of the background galaxies," Jee explained.
"It's like looking at the pebbles on the bottom of a pond
with ripples on the surface. The pebbles' shapes appear to
change as the ripples pass over them. So, too, the
background galaxies behind the ring show coherent changes
in their shapes due to the presence of the dense ring."
Jee and his colleagues used Hubble's Advanced Camera
for Surveys to detect the faint, distorted, faraway
galaxies behind the cluster that cannot be resolved with
ground-based telescopes.
"Hubble's exquisite images and unparalleled
sensitivity to faint galaxies make it the only tool for
this measurement," said team member Richard White of the
Space Telescope Science
Institute in Baltimore.
Previous observations of the Bullet Cluster with
Hubble and the Chandra X-ray Observatory presented a
sideways view of a similar encounter between two galaxy
clusters. In that collision, the dark matter was pulled
apart from the hot cluster gas, but the dark matter still
followed the distribution of cluster galaxies. Cl 0024+17
is the first cluster to show a dark matter distribution
that differs from the distribution of both the galaxies and
the hot gas.
The team's paper will appear in the June 1 issue of
the Astrophysical Journal.