The Mother of All Pileups

17 April 2009 (All day)

NASA/CXC/STScI/IfA/C. Ma et al.

Gridlock. This composite image shows hot gas (blue) from four galaxy clusters converging along a cosmic filament.

In a faraway corner of the universe, a crash of cosmic proportions is under way, cramming more than 1000 galaxies into a space normally reserved for a handful. It's also compressing and heating enormous quantities of intergalactic gas. Astronomers studying the phenomenon say what they learn about the pileup should improve their understanding about how the largest structures in the cosmos have evolved.

Our solar system sits in a quiet corner of a large, but otherwise ordinary--and solitary--galaxy. But in many other parts of the universe, things aren't so quiet. Collisions between galaxies and even clusters of galaxies are surprisingly routine. But the event called MACSJ0717 is a horse of a different color. Discovered in 2003 and located about 5.4 billion light-years away, it involves four distinct clusters of galaxies converging along a filament of galaxies, gas clouds, and, mainly, dark matter that's about 13.5 million light-years long. (For comparison, the distance between our Milky Way and its nearest neighbor, the Andromeda Galaxy, is about 3 million light-years.) Such filaments are thought to permeate the universe and to draw in galaxies and intergalactic gas clouds from less dense regions.

It took a while to identify the filament, explains astronomer and co-author Harald Ebeling of the University of Hawaii, Honolulu. At first, he says, x-ray surveys of the clusters "showed just a mess, and the optical image didn't look much better." The problem, Ebeling says, was that the system of galaxies did not have a well-defined center, and the features in the x-ray images, which showed the distribution of hot gas, didn't seem to match the distribution of the galaxies. "We scratched our heads for a long time over this one," he says. "What exactly was going on here?"

Further observations by lead researcher Cheng-Jiun Ma provided the critical clue: The temperatures of the constituent gas clouds--whose collective mass far outweighs the galaxies--suggested that the researchers were looking at multiple clusters colliding. "The pieces began to fall into place," says Ebeling, who with Hawaii colleagues Ma and Elizabeth Barrett published the findings last month in The Astrophysical Journal and released a composite image of MACSJ0717 yesterday. Ebeling says Ma's comparisons of the temperature peaks of the gas and the galaxy distributions enabled the team to deduce the directions of motion of the clusters.

"Voilà!" Ebeling says. "Not only did a consistent 3D picture of this wild merger appear, but it also pointed conclusively toward the filament as the common source of all the mayhem." That discovery means theorists have been right about the role of filaments in funneling matter in the visible universe. If confirmed, Ebeling says, the filament shaping MACSJ0717 could provide astronomers with a unique opportunity to measure the key properties of these cosmic highways.

Finding a triple merger is extremely rare, says astrophysicist Craig Sarazin of the University of Virginia in Charlottesville. It will help to measure the mass of dark matter and its level of influence on visible matter, he says. Though researchers have seen smaller versions of such mergers before, says astrophysicist Mike Hudson of the University of Waterloo in Canada, with this one investigators will have to "disentangle the wreckage and figure out exactly what happened." If it can be done, he says, it should yield "important clues to cluster and galaxy formation."

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