stubborn, invisible stuff that makes up nearly a quarter of the mass and energy of the universe, but refuses to interact with ordinary matter except through gravity. The only way to know dark matter is there at all is by observing how its mass warps and tugs at visible matter.

When a lot of dark matter clumps together, as in massive galaxy clusters that contain hundreds or thousands of galaxies, it can act as an enormous magnifying glass for even more distant galaxies. The cluster’s gravity stretches and distorts the light from galaxies behind it like a fun house mirror. Astronomers on Earth see multiple warped images of each galaxy, a phenomenon called gravitational lensing.

Gravitational lensing can give a good idea of how much dark matter is in a cluster, but up until now astronomers had to guess at where exactly the dark matter was.

Now, using an image from Hubble’s Advanced Camera for Surveys, astronomers have built a high-resolution map of exactly where the dark stuff lurks in a galaxy cluster called Abell 1689.

“Other methods are based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data,” said astronomer Dan Coe of NASA’s Jet Propulsion Laboratory in a press release. “Using our method, we can obtain, directly from the data, a mass map that gives a perfect fit.”

Abell 1689 lies 2.2 billion light-years away and contains about 1,000 galaxies and trillions of stars. By combining the Hubble image with earlier observations, astronomers picked out 135 multiple images of 42 background galaxies.

“The lensed images are like a big puzzle,” Coe said. “Here we have figured out, for the first time, a way to arrange the mass of Abell 1689 such that it lenses all of these background galaxies to their observed positions.”

Coe and colleagues superimposed the locations of dark matter in the cluster (shown in blue, above) onto the Hubble image. The results, which appear in the Nov. 10 Astrophysical Journal, confirmed that Abell 1689 has more dark matter packed closer together than astronomers expected for a cluster its size.

That extra bulk could indicate that galaxy clusters formed earlier in the history of the universe than astronomers thought. Dark matter’s gravity pulls matter together, but it’s countered by another, even more mysterious force called dark energy, which pushes matter apart. Once dark energy became an important player in the early universe, galaxy clusters would have had a hard time sticking together.

“Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today,” Coe said. “At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”

More data is still to come from a project called CLASH (Cluster Lensing And Supernova survey with Hubble), which will aim Hubble at 25 galaxy clusters for a total of one month over the next three years.

Image: NASA, ESA, D. Coe (NASA Jet Propulsion Laboratory/California Institute of Technology, and Space Telescope Science Institute), N. Benitez (Institute of Astrophysics of Andalusia, Spain), T. Broadhurst (University of the Basque Country, Spain), and H. Ford (Johns Hopkins University)

See Also:

• Farthest Galaxy Cluster Ever Detected
• Monster Galaxy Cluster Found in the Distant Universe
• Google Sky Adds Galaxy Clusters
• Cold, Dead Stars Could Help Limit Dark Matter
• Signs of Destroyed Dark Matter Found in Milky Way’s Core
• Mapping Dark Matter’s Invisible Super-Clumps

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Authors: Lisa Grossman

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