Fleet finding: speed of Milky Way's companions poses puzzle.

Visible to the naked eye, the Large and Small Magellanic Clouds are two of the Milky Way's closest companion galaxies. Scientists have assumed that these groups of stars have been orbiting the Milky Way for billions of years. But new measurements of the speed of these familiar fixtures now put astronomers in unfamiliar territory. Either the two tiny galaxies are just whizzing by or our galaxy is twice as massive as many scientists had estimated.

To record the motion of the clouds of stars as they inch across the sky, Nitya Kallivayalil of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues used the Hubble Space Telescope. This velocity is notoriously difficult to detect because the galaxies appear to barely change position from year to year.

Kallivayalil and her collaborators accurately discerned that motion by comparing the changing positions of the two galaxies over a 4-year interval with the location of distant quasars that lie behind the clouds and provide fixed points of reference.

When the researchers combined the observed velocities with previous measurements of the galaxies' motion along the line of sight, they concluded that the galaxies are traveling at about twice the speed previously estimated. The Large Magellanic Cloud streams through space at 378 kilometers per second, while the Small Magellanic Cloud moves at 302 km/see. These speeds are about 200 times that of a bullet.

The group announced the findings on Jan. 9 at a meeting of the American Astronomical Society in Seattle. The new work agrees with Hubble measurements reported in the March 2006 Astronomical Journal by another group of researchers, led by Mario H. Pedreros of the University of Tarapaca in Arica, Chile.

The satellite galaxies couldn't be moving that fast if they were circling the Milky Way as it's currently understood, Kallivayalil says. She suggests three possible explanations. In one scenario, the speedy Magellanie Clouds wouldn't orbit our galaxy but instead would leave our cosmic neighborhood in a few billion years.

Another possibility is that the Milky Way is about twice as heavy--about 1 trillion solar masses--as the current estimate, providing an additional gravitational grip that would keep the clouds in orbit. Like most of the Milky Way's weight, this mass would be in dark matter--material that exerts a tug but can't be seen.

The speed of the clouds might also be explained if the Milky Way's dark matter halo is lopsided, with more material concentrated along one direction than another, Kallivayalil says.

Theorist Doug Lin of the University of California, Santa Cruz says that he favors a more massive Milky Way as the explanation of the velocity measurements, which match values that he and a colleague predicted in 1982.

Doubling the mass of the Milky Way by adding dark matter wouldn't greatly change astronomer's views of the galaxy's structure and interactions, he says. Moreover, Lin says, a long streamer of hydrogen gas that trails the Magellanie Clouds can best be explained if it was torn out by the Milky Way's gravity, another indication that the clouds are indeed orbiting the galaxy.