Big smash: galaxy clusters in collision.

Astronomers last week unveiled the most detailed image ever taken of a rare type of cosmic train wreck--the collision of two of the universe's most massive clusters of galaxies. Taking place over several hundred million years, the merger is the most powerful that's ever been so clearly recorded, says Patrick Henry of the University of Honolulu in Hawaii. He and his collaborators described the findings during a NASA telephone briefing on Sept. 23 and will report additional details in an upcoming Astrophysical Journal.

Henry and his colleagues used the European Space Agency's XMM-Newton Telescope to document the X-ray emissions from hot gas in the cluster Abell 754. Containing about 1,000 galaxies, Abell 754 is one of the massive clusters nearest to Earth, residing 800 million light-years away. Astronomers estimate that only a few hundred massive clusters exist simultaneously in the cosmos and that only 20 to 30 of them are in the midst of collisions.

"We've seen a lot of [cluster] systems that are obviously interacting--with tidal streams, dozens of little things falling into much bigger things--but ... surprisingly few blatant head-on crashes of two seriously large clusters," comments Megan Donahue of Michigan State University in East Lansing.

Abell 754 was already known to be merging with a lower-mass system, which has about 300 galaxies. But with the new X-ray image, astronomers can discern details of the collision, including shock waves of gas at 100 million kelvins plowing through intergalactic space. That finer view has enabled the scientists to construct the equivalent of a weather map showing the distributions of pressures and temperatures within the massive collision site.

The hot intergalactic gas, which is 10 times as massive as all the stars and gas within the clusters' galaxies combined, bears the brunt of the collision and can best be traced by X-ray observations, notes Henry. An analysis of the ongoing collision suggests that the two clusters were distinct until about 300 million years before the epoch captured in the images, he adds.

That analysis also suggests that the smaller cluster has plowed through the bigger one, but that gravity will next pull the remnants of the smaller cluster back toward the core of the bigger one. In a few billion years, the disturbance will die down and the merger will be complete.

The mammoth merger supports the idea that the overall structure in the universe emerged from the bottom up, with smaller objects gravitationally coalescing to make bigger ones, notes Henry.

The X-ray observations also agree with simulations in which an invisible, exotic material known as dark matter makes up more than 80 percent of the mass of Abell 754 and is primarily responsible for pulling together the clusters, says Gus Evrard of the University of Michigan in Ann Arbor.

That agreement "indicates that we are on the right track in our modeling of the universe," comments Richard Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md.