What is Dark Matter?
Astronomers are using NASA's Hubble Space Telescope to dissect one of the largest
structures in the universe as part of a quest to understand the violent lives of galaxies.
Hubble is providing indirect evidence of unseen dark matter tugging on galaxies in the
crowded, rough-and-tumble environment of a massive supercluster of hundreds of galaxies.
Dark matter is an invisible form of matter that accounts for most of the universe's
mass. Hubble's Advanced Camera for Surveys has mapped the invisible dark matter
scaffolding of the supercluster Abell 901/902, as well as the detailed structure of
individual galaxies embedded in it.
The images are part of the Space Telescope Abell 901/902 Galaxy Evolution
Survey (STAGES), which covers one of the largest patches of sky ever observed
by the Hubble telescope. The area surveyed is so wide that it took 80 Hubble images
to cover the entire STAGES field. The new work is led by Meghan Gray of the University
of Nottingham in the United Kingdom and Catherine Heymans of the University of British
Columbia in Vancouver, along with an international team of scientists.
Building a Dark Matter Map
The Hubble study pinpointed four main areas in the supercluster where dark matter has
pooled into dense clumps, totaling 100 trillion times the Sun's mass. These areas match
the location of hundreds of old galaxies that have experienced a violent history in their
passage from the outskirts of the supercluster into these dense regions. These galaxies
make up four separate galaxy clusters.
"Thanks to Hubble's Advanced Camera for Surveys, we are detecting for the first time
the irregular clumps of dark matter in this supercluster," Heymans said. "We can
even see an extension of the dark matter toward a very hot group of galaxies that are
emitting X-rays as they fall into the densest cluster core."
The Galaxy Environment
The dark matter map was constructed by measuring the distorted shapes of over 60,000
faraway galaxies. To reach Earth, the galaxies' light traveled through the dark matter
that surrounds the supercluster galaxies and was bent by the massive gravitational field.
Heymans used the observed, subtle distortion of the galaxies' shapes to reconstruct the
dark matter distribution in the supercluster using a method called weak gravitational lensing.
The dark matter map is 2.5 times sharper than a previous ground-based survey of the supercluster.
"The new map of the underlying dark matter in the supercluster is one key piece of this
puzzle," Gray explained. "At the same time we're looking in detail at the galaxies
themselves." The survey's broader goal is to understand how galaxies are influenced by the
environment in which they live.
On Earth, the pace of quiet country life is vastly different from the hustle of the big city.
In the same way, galaxies living lonely isolated lives look very different from those found in
the most crowded regions of the universe, like a supercluster. "We've known for a long time
that galaxies in crowded environments tend to be older, redder, and rounder than those in the
field," Gray said. "Galaxies are continually drawn into larger and larger groups and
clusters by the inevitable force of gravity as the universe evolves."
Galaxy Collisions
In such busy environments galaxies are subject to a life of violence: high-speed collisions
with other galaxies; the stripping away of gas, the fuel supply they use to form new stars; and
distortion due to the strong gravitational pull of the underlying invisible dark matter. "Any
or all of these effects may play a role in the transformation of galaxies, which is what we're
trying to determine," Gray said.
The STAGES survey's simultaneous focus on both the big picture and the details can be likened
to studying a big city. "It's as if we're trying to learn everything we can about New York
City and New Yorkers," Gray explained. "We're examining large-scale features, like
mapping the roads, counting skyscrapers, monitoring traffic. At the same time we're also studying
the residents to figure out how the lifestyles of people living downtown differ from those out
in the suburbs. But in our case the city is a supercluster, the roads are dark matter, and the
people are galaxies."
Further results by other team members support this view. "In the STAGES supercluster we
clearly see that transformations are happening in the outskirts of the supercluster, where
galaxies are still moving relatively slowly and first feel the influence of the cluster
environment," said Christian Wolf, an Advanced Research Fellow at the University of Oxford
in the U.K.
Assistant professor Shardha Jogee and graduate student Amanda Heiderman, both of the
University of Texas in Austin, concur. "We see more collisions between galaxies in the
regions toward which the galaxies are flowing than in the centers of the clusters," Jogee
said. "By the time they reach the center, they are moving too fast to collide and merge, but
in the outskirts their pace is more leisurely, and they still have time to interact."
The STAGES team also finds that the outer parts of the clusters are where star formation in
the galaxies is slowly switching off and where the supermassive black holes at the hearts of
the galaxies are most active.
Added Heiderman: "The galaxies at the centers of the clusters may have been there for a
long time and have probably finished their transformation. They are now old, round, red, and
dead."
The team plans more studies to understand how the supercluster environment is
responsible for producing these changes.
Abell 901/902 resides 2.6 billion light-years from Earth and measures more than 16 million
light-years across.
Gray and Heymans presented their findings on in January 2008 at the 211th meeting of the
American Astronomical Society in Austin, Texas. A science paper on their results has been
accepted by the Monthly Notices of the Royal Astronomical Society.
This work was supported by the Science and Technology Facilities Council (UK), NASA, the
National Science Foundation Long Term Space Astrophysics (NASA LTSA) program, a Marie Curie
Fellowship, a CITA National Fellowship, CIfAR, and CFI.
This information above and the images were published as a NASA news release in January, 2008.
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Dark Matter Map: Astronomers assembled this photo by combining a visible-light image
of the Abell 901/902 supercluster taken with the MPG/ESO 2.2-meter
telescope in La Silla, Chile, with a dark matter map derived from
observations with NASA's Hubble Space Telescope. The magenta-tinted
clumps represent a map of the dark matter in the cluster. Dark matter
is an invisible form of matter that accounts for most of the universe's
mass. The image shows that the supercluster galaxies lie within the
clumps of dark matter.
Hubble cannot see the dark matter directly. Astronomers inferred its
location by analyzing the effect of so-called weak gravitational lensing,
where light from more than 60,000 galaxies behind Abell 901/902 is distorted
by intervening matter within the cluster. Researchers used the observed,
subtle distortion of the galaxies' shapes to reconstruct the dark matter
distribution in the supercluster.
Credit: NASA, ESA, C. Heymans (University of
British Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M.
Barden (Innsbruck), the STAGES collaboration, C. Wolf (Oxford University,
U.K.), K. Meisenheimer (Max-Planck Institute for Astronomy, Heidelberg),
and the COMBO-17 collaboration
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Abell 901a.Image Credit: NASA, ESA, C. Heymans (University of British
Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M.
Barden (Innsbruck), and the STAGES collaboration.
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Abell 901b.Image Credit: NASA, ESA, C. Heymans (University of British
Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M.
Barden (Innsbruck), and the STAGES collaboration.
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Abell 902. Image Credit: NASA, ESA, C. Heymans (University of British
Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M.
Barden (Innsbruck), and the STAGES collaboration.
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SW Group. Image Credit: NASA, ESA, C. Heymans (University of British
Columbia, Vancouver), M. Gray (University of Nottingham, U.K.), M.
Barden (Innsbruck), and the STAGES collaboration.
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