for National Geographic News
In galaxy clusters, the rough-and-tumble outer suburbs are where most galactic change occurs, a team of astronomers announced today.
There, galaxies harass, strangle, and strip away at each other as they are pulled from the outskirts of a cluster to the inner core by the gravity of dark matter, according to the new research.
The finding stems from detailed analysis of a massive supercluster of more than a thousand galaxies that lies about 2.6 billion light-years away from Earth.
Catherine Heymans, an astrophysicist at the University of British Columbia in Vancouver, Canada, used a mosaic of 80 Hubble images spanning 16 million light-years to create a map of dark matter in the supercluster.
"This is the highest resolution of dark matter of one of the largest areas ever imaged by the Hubble Space Telescope," she said during a press briefing today at the 211th meeting of the American Astronomical Society (AAS) in Austin, Texas. (Related news: "Dark Matter Mapped in 3-D, Scientists Report" [January 8, 2007].)
Dark matter does not emit light, making it invisible. But based on long-standing theories of gravity, scientists think dark matter makes up the vast majority of mass in the universe.
The dark matter in the visible supercluster Abell 901/902 bends the light of about 60,000 galaxies that lie further behind it. This effect, called weak gravitational lensing, allowed Heymans to infer the presence of dark matter in the supercluster.
"Imagine looking through a window," Heymans said. Since the glass is clear, "how do you know the window is there? Now imagine raindrops on the window—that distorts my view."
Similar distortion in the light from the distant galaxies allowed the researchers to create the new map, which shows four areas where dark matter has clumped together in the supercluster.
Each clump matches the known location of hundreds of galaxies.
Meghan Gray, an astrophysicist at the University of Nottingham in the United Kingdom, observed these clusters of galaxies in various wavelengths of light to understand how dark matter influences galactic evolution.
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