Separation anxiety: cosmic collision may shed light on dark matter.

Some 3 billion years ago, two massive clusters of galaxies collided head on. The debris from this ancient cosmic train wreck, astronomers say, might pose a new puzzle about the invisible material believed to account for most of the mass in the universe.

A variety of evidence indicates that this material, known as dark matter, is about eight times as abundant as ordinary matter and that it resides in vast, invisible halos around star-filled galaxies. Dark matter keeps galaxies and galaxy clusters intact, theorists say.

The leading model for dark matter suggests that it interacts only through gravity and can't be pushed around by the strong, the weak, or electromagnetic forces. That picture gained support last year from observations of a collection of galaxies called the Bullet cluster, which had been distorted by a collision with another cluster. Astronomers inferred that the location of dark matter coincided with the cluster's visible horde of galaxies, while the hot, X-ray-emitting gas associated with the cluster lay to one side (SN: 8/26/06,p. 131).

That distribution of dark matter makes sense because colliding gas clouds interact both by gravity and the electromagnetic force and can slow each other down, while dark matter and galaxies would breeze along unimpeded and remain together.

That's why new X-ray and visible-light observations of the cluster Abell 520 stunned Andisheh Mahdavi of the University of Victoria in British Columbia and his colleagues. The cluster, about 2.5 billion light-years from Earth, had also suffered a major collision.

In the Oct. 20 Astrophysical Journal, the team reports that Abell 520 contains concentrations of dark matter and galaxies that are separate from each other.

"The Bullet cluster was a spectacular result, because it beautifully confirmed our assumptions about how dark matter, gas, and galaxies behave, [but] Abell 520 does the complete opposite," comments Julianne Dalcanton of the University of Washington in Seattle.

One explanation for the new results is that dark matter is composed of particles that interact through forces other than gravity. However, such particles would cause a variety of other effects that have never been seen, such as making galaxy clusters spherical, notes Katherine Freese of the University of Michigan in Ann Arbor. Without further evidence, theorists seem loath to reject the standard take on dark matter.

The standard view could prevail if galaxies had been ejected from the core of Abell 520. That might have occurred if the cluster had suffered more than one collision, Mahdavi says.

It's also possible that Mahdavi's team, along with two other groups also using ground-based telescopes, didn't have data precise enough to correctly map the dark matter in Abell 520. To map dark matter, researchers measure the distortion of images of background galaxies whose light passes through the cluster on its way to Earth. That's a tricky business, because galaxies are naturally elongated, notes Douglas Clowe of Ohio University in Athens, who used the sharper eye of the Hubble Space Telescope to examine the Bullet cluster.

In their own ground-based study, Clowe and his collaborators don't find a significant separation between dark matter and galaxies in Abell 520, Clowe told Science News. That's in accord with the results of another team, which reported its results earlier this year online (http://xxx.lanl.gov/ abs/astro-ph/0702649).

Upcoming observations of Abell 520 with Hubble should indicate whether dark matter theory really has to be reassessed or if researchers are merely arguing about noisy data, says Clowe.