Interactions News Wire #27-07
15 May 2007
http://www.interactions.org
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Source: Johns Hopkins University
Content: Press Release
Date Issued: 15 May 2007
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Johns Hopkins Team Finds Ring of Dark Matter
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 The Johns Hopkins University.
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 team's 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 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 galaxies 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.
CONTACT: Lisa De Nike, JHU
(443) 287-9960; Lde@jhu.edu
Donna Weaver/Ray Villard, STScI
410) 338-4493/4514
Grey Hautaluoma/Tabatha Thompson
NASA Headquarters, (202) 358-0668/3895
For images and more information about the dark matter ring, see:
http://hubblesite.org/news/2007/17
http://www.nasa.gov/hubble
http://www.spacetelescope.org/news/html/heic0709.html
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope Science
Institute conducts Hubble science operations. The institute is operated
for NASA by the Association of Universities for Research in Astronomy Inc.
in Washington.
