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Crash course. A simulation suggests that the gravity of hidden dark matter dictates the orbits of stars after galaxies collide, including stars flung far into space on distant, slow-moving orbits (right panel).

Dark Matter Survives Another Test

The cosmic lost-and-found department now contains an important "found" item: dark matter. A new simulation of the orbits of stars after galaxy collisions concludes that invisible cocoons of matter do indeed exist around large nearby galaxies. The study refutes a recent claim that some of those galaxies have little or no dark matter--a provocative idea that flew in the face of leading theories of galaxy formation.

In the earlier research astronomers examined nebulae of gas in the outskirts of three giant blobs of stars called elliptical galaxies. The nebulae form when dying stars similar to our sun expel their atmospheres. A team led by Aaron Romanowsky, now at the Universidad de Concepción in Chile, found that about 100 nebulae in each galaxy seemed to orbit very slowly. The researchers maintained that if the galaxies held far more dark matter than visible stars, then the nebulae should orbit faster under the matter's gravitational influence.

But there's no need to rewrite the theories of galaxy formation, says astrophysicist Avishai Dekel of Hebrew University in Jerusalem, Israel. Dekel and coworkers scrutinized the mergers between big spiral galaxies that probably form elliptical galaxies. Computer models showed that the collisions fling arcs of stars deep into space. The team's virtual galaxies included dark matter, which pulled most of the stars back toward the merged galaxies and forced them into swooping, cigar-shaped paths.

The model matches the observations of slow-moving nebulae by Romanowsky's team, Dekel says, because telescopes would see most of the nebulae at the farthest and slowest points of their long orbits. The simulation also suggests that the nebulae actually move through space more quickly than Romanowsky's team deduced, because telescopes can only measure the component of their motions directly toward or away from Earth. Faster motions toward the sides of the galaxy would not have shown up, says Dekel, whose team describes its findings 29 September in Nature.

"This is a plausible and simple interpretation," says astrophysicist Julio Navarro of the University of Victoria, British Columbia. "It demonstrates that with just 100 [nebulae] for each galaxy, it's hard to rule out well-established dark matter scenarios." Romanowsky calls the work by Dekel's team "important," but he adds a caveat: "It's not clear their model explains everything we see." Ongoing analysis of nebulae in 12 elliptical galaxies still points to less dark matter than expected, he says.

Related sites
Abstract of Dekel et al. paper, with link to full text
Planetary Nebula Spectrograph, used in Romanowsky's studies
An introduction to dark matter