New Dark Matter Map Shows Violent Life of Galaxies

John Roach
for National Geographic News
January 10, 2008
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.

Suburban Influence

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.

"It's as if we're listening to a symphony and we want to make sure we listen to all different parts of the orchestra," she said of the multiwavelength study.

"If we limit ourselves to one wavelength—if we only listen to the wind section—we don't see the full picture of our studies of galaxy evolution."

The full analysis shows that galaxies undergo violent changes as they are pulled by the attractive force of gravity through the outskirts of the clusters toward the dense cluster cores.

"As these galaxies move in from the country to the big city, it's in the suburbs where we see them in the process of change," she said.

For example, some galaxies are stretched as they quickly slide past each other. Others have their gas slowly sucked out by the gravity of another, rendering them unable to make new stars.

"There're even more violent processes, like stripping: The gas is pulled out all together as they plunge into these dense regions," Gray said.

Gray noted that the galaxies in the center of the clusters look different—they're older, deader, and less dense.

Perhaps, she said, the central galaxies move so quickly in their orbits that they are unable to attract each other and collide as they do in the suburbs.

The researcher noted that the highly detailed map of Abell 901/902 allowed them to catch the galaxies at just the right moment to see these changes. But results from other teams studying the same supercluster seem to back up the findings.

"We see more collisions between galaxies in the regions toward which the galaxies are flowing than in the centers of the clusters," Shardha Jogee of the University of Texas in Austin commented in a press release.

Jogee presented unrelated research at the AAS meeting that used images from space-based telescopes to show that galaxies in the local universe go through a "mid-life crisis" and stop merging as frequently and violently as they did when the universe was young.

By the time galaxies reach the center of their clusters, Jogee said, "they are moving too fast to collide and merge, but in the outskirts their pace is much more leisurely, and they still have time to interact."

(Victoria Jaggard in Austin, Texas, contributed to this report.)

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