Sept. 20, 2007 — Our closest galactic companions are not companions at all — just strangers passing us in the night. New measurements of the speeds of the Magellanic Clouds show that the cloudy apparitions of Southern skies may be zooming past the Milky Way and not its little siblings after all.

New 3-D measurements of both the Large Magellanic Cloud and the Small Magellanic Cloud put their speeds at close to the escape velocity from the Milky Way. That suggests they're rather new arrivals and may not even stick around.

The speeds of the lesser galaxies were worked out by first establishing the precise positions of very distant and powerful quasars in the deep background sky behind the vastly closer Magellanic Clouds. With the quasars providing an unmoving frame of reference, observations with the Hubble Space Telescope allowed astronomers to directly measure changes in positions of stars in the Magellanic Cloud.

The high speeds makes sense, said astronomer Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics, because the Magellanic Clouds never really looked like they come from around here. They have, for instance, too much gas in them, she said. Gas is the first thing to be ripped away when galaxies play tug-o-war. In such cases the biggest galaxy usually gets the gas.

"They don’t look like they’ve had much interaction with the Milky Way," said Besla, the lead author of a report on the discovery that’s slated for publication in Astrophysical Journal later this year. In fact, this very well could be the first time both of the dwarf galaxies have been so close to our galaxy, she told Discovery News. So in other words, the newly measured speeds make a lot of sense, she said.

The new speeds also jive with another line of evidence: theoretical modeling of the Magellanic Clouds and the Milky Way.

"Our simulations suggest that satellite galaxies as large as the Magellanic Clouds should be destroyed very quickly after they fall within reach of the Milky Way," explains galactic modeler James Bullock of Center for Cosmology at the University of California at Irvine. By quickly he means within about 3 billion years.

"In this case, it would be surprising if they were not on their first passage," said Bullock. "If they had fallen in longer ago, we wouldn't expect them to be around any more. In this sense, Besla's results are in accord with theoretical expectations for galaxies like these."

Exactly where the Magellanic Clouds are going and where they have been is harder to say, Besla said. It’s a lot more complicated than calculating the orbit of a planet around a star, although the physics are basically the same.

In this case there are the two Magellanic Clouds, the Milky Way, Andromeda and a few other friends and neighbors flying around in what’s called the Local Group.

As for why the discovery comes only now, it’s mostly a matter of having the technology to make accurate measurements of the Magellanic Clouds’ speeds without huge error bars, Besla explained. "There’s really never been anything that ruled this out before," she said.