Motion of two nearby galaxies clouds the picture

Astronomers may have to rewrite the textbooks on the two galaxies closest to the Milky Way. A new analysis confirms previous indications that the Large and Small Magellanic Clouds are not gravitationally bound to our galaxy but are instead speeding by, having come this way just 1 billion to 3 billon years ago.

Visible to the naked eye as bright clouds, the two galaxies are small and irregular. The Large Magellanic Cloud is about 160,000 light-years from the Milky Way and has about one-twentieth the Milky Way’s diameter, while the Small Magellanic Cloud, one-200th the Milky Way’s size, lies about 200,000 light-years distant.

Earlier this year, Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues measured the velocities of the galaxies relative to the Milky Way with unprecedented accuracy. The velocities were unexpectedly high, suggesting that either the two galaxies aren’t bound to the Milky Way or that our galaxy is much heavier and exerts a greater gravitational tug than astronomers had estimated (SN: 1/13/07, p. 19). Further analysis of those data now shows that the two galaxies are traveling along parabolic orbits, meaning that each is making its first pass by the Milky Way, the team reports in the Oct. 20 Astrophysical Journal.

That’s a puzzle on several counts, notes Besla. For instance, it’s not clear what pulled the Magellanic Stream, a long tail of hydrogen gas, out of the two clouds. Some astronomers had suggested that gravitational interactions between the clouds and the Milky Way created the trail. Alternatively, the trail could have arisen if the two galaxies had rammed into the outskirts of the Milky Way. But the new findings show that the clouds haven’t been hanging around the Milky Way long enough for either scenario to have happened.

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