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"Long before it's in the papers"
August 03, 2010

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Wall of galaxies tugs on ours, astronomers find

April 19, 2006
Special to World Science

Researchers say they’ve filled in a key piece of the map of our local universe, by discovering a mighty wall of galaxies that may be the “Great Attractor” astronomers have been trying to identify for years.

Part of a cluster of galaxies called ACO 3627 near the center of the Great Attractor. (Credit: 2P2 Team, WFI, MPG/ESO 2.2-m Telescope, La Silla, ESO) 

The “Great Attractor” is a distant, mysterious entity that seems to be tugging tens of thousands of galaxies, including ours, rapidly toward itself.

The new findings suggest these motions are the result of gravitational forces from not one, but two things:
the wall, and a conglomerate of galaxies far beyond it.

It seems “roughly 50% of our galaxies’ motion through space is due to [the wall] and about 50% is due to structures behind it,” wrote Dale Kocevski of the University of Hawaii in an email. Kocevski is a member of one of the research teams that reported the findings. 

Astronomers have known for years that something seems to be pulling our Milky Way and other galaxies toward itself at a breakneck 22 million kilometers (14 million miles) per hour.

But they couldn’t before now pinpoint exactly what or where it is.

Although it’s tugging on us, we’ll never reach it, said David Radburn-Smith the University of Durham, U.K., whose team identified the “wall.” That’s because the expansion of the universe is stretching the wall’s neighborhood away from ours about nine times faster than the speed with which gravity is drawing them together. The stretching effect would be still swifter for further objects.

Radburn-Smith and colleagues described the “wall” in a new paper accepted for publication in the research journal Monthly Notices of the Royal Astronomical Society. It “does appear to be a wall-like slab of galaxies,” he wrote in an email, though its precise shape is “tricky to define” because the dust of our Milky Way galaxy obscures much of it.

Radburn-Smith, a Ph.D. student, is the paper’s lead author.

He added that the wall contains the weight equivalent of some 12,000 Milky Way galaxies, and is around 200 million light-years away. A light-year is the distance light travels in a year.

The wall seems to sweep over an angle of about 100 degrees near the top of the Southern Hemisphere sky, the astronomers wrote; this distance corresponds to some 400 million light-years. One end would be roughly in the direction the star Mu Velorum in the constellation Vela, the other in the vicinity of Al Dhanab in the constellation Grus.

In between, the structure curves into the silvery strip of the Milky Way, they reported, where it merges with a cluster of galaxies called Norma—roughly in the direction of the star Beta Trianguli Australis in the Southern Triangle constellation. One member of the team, Patrick Woudt of the 3University of Cape Town, South Africa, proposed previously that Norma marks the core concentration of the Great Attractor’s mass. 

The researchers drew their results from an array of galactic distance measurements based on redshift—the reddening of light from galaxies. Further-off galaxies are redder because as the universe expands, it pulls objects apart from each other, “stretching” light waves traveling between them. The greater the distance between objects, the stronger the effect.

Surveys of the universe at its largest scales have found that galaxies are arranged into a sponge-like structure, with sheets and filaments of galaxies surrounding nearly empty voids. Places where these sheets and filaments intersect are sometimes called “knots,” as they tend to have dense concentrations of galaxies that are merging.

Radburn-Smith said his findings help clarify our place in this sort of structure.

The Milky Way and its neighboring Andromeda galaxy, along with some 30 smaller ones, “form what is known as the Local Group,” he explained in an email. This lies on the outskirts of a “supercluster”—a grouping of thousands of galaxies—known as Virgo, which is also pulled toward the Great Attractor.

The Virgo Supercluster is centered on a “knot,” he added. The Local Group lies on a broad filament protruding from this knot. Another filament also branches off from it—at right angles to ours—and extends to a second knot, known as the Centaurus cluster, he added. 

From there, yet another filament stretches toward a third knot, the “Norma Cluster,” which is part of the Great Attractor wall, he explained. “There’s no direct connection between our galaxy and the Great Attractor.”

Astronomers have previously found other sheet-like conglomerations of galaxies described as “Great Walls.” This newfound structure may be similar, Radburn-Smith suggested. 

But Kocevski said his own work shows the wall and associated structures lack enough mass to provide the gravitational pull hitherto attributed to the Great Attractor. Thus, he proposes that more mass lies beyond the wall.

In findings presented Jan. 11 at the American Astronomical Society meeting in Washington, D.C., Kocevski, also a doctoral student, and other researchers at his institution said a major concentration of galaxies lies beyond the Great Attractor. They’re near the so-called Shapley Supercluster, 500 million light-years away—the most massive known supercluster.

Kocevski wrote in an email that his and Radburn-Smith’s findings could both be correct; in fact, “our work mapping X-ray luminous galaxy clusters in the Great Attractor region has reached the same conclusion” as Radburn-Smith. “The pull our galaxy is feeling is most likely due to both the nearby Great Attractor and these more distant structures.”

The researchers are using the name “Great Attractor” only for the wall and related structures, not these much further objects. The naming is in line with past practice: astronomers had long suspected the Great Attractor lay in the neighborhood now being fingered as the abode of the wall. Thus they called that zone, but not the area behind it, the “Great Attractor region.”

This article has been corrected since its initial posting: The Great Attractor is believed to be pulling tens of thousands of galaxies toward itself, not millions as originally stated.

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WORLD SCIENCE

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