THE Milky Way鈥檚 two best-known companions may be nothing more than strangers passing by. Recent observations of the Magellanic Clouds, a pair of nearby dwarf galaxies, reveal that they are moving too fast to be satellites of the Milky Way 鈥� unless our galaxy contains twice as much dark matter as thought.
鈥淭hey are moving too fast to be satellites of the Milky Way, unless our galaxy has twice as much dark matter as thought鈥�
Astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and the Space Telescope Science Institute (STScI) in Baltimore, Maryland, compared Hubble Space Telescope images taken two years apart to make the most accurate measurements to date of the velocities of the Magellanic Clouds. During the two years, both galaxies had shifted a minuscule amount, about one-hundredth the width of one pixel in Hubble鈥檚 field of view. The astronomers detected these tiny shifts by carefully aligning the clouds in each pair of images and then measuring the apparent change in the positions of several background quasars.
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The astronomers used this information to calculate the clouds鈥� true velocities through space, not just across our line of sight. This yielded figures of 378 kilometres per second and 302 km/s for the Large and Small Magellanic clouds respectively. If the two galaxies were orbiting the Milky Way 鈥� as many astronomers still believe 鈥� their velocities would have to be about 250 km/s. 鈥淚 really wasn鈥檛 expecting them to be moving so fast,鈥� says CfA鈥檚 Nitya Kallivayalil.
If the clouds are not gravitationally bound to the Milky Way, astronomers will be hard pressed to explain the Magellanic stream 鈥� a river of neutral hydrogen gas that trails behind the Magellanic system and extends across more than a quarter of the night sky. 鈥淭he stream implies a bound orbit,鈥� says Martin Weinberg of the University of Massachusetts, Amherst.
The alternative explanation, that the Milky Way contains far more dark matter than expected and so causes the dwarfs to orbit more rapidly, would be equally surprising but may prove easier to accommodate.
Kallivayalil and her colleagues are also investigating whether a clumpy or otherwise asymmetrical distribution of dark matter within the Milky Way could account for the clouds鈥� speedy motion. If such models prove unsuccessful, however, 鈥渢hen maybe we鈥檒l have to say the clouds are not bound,鈥� says Roeland van der Marel of STScI.
In that case, says Kallivayalil, sky watchers on Earth will have approximately 3 billion years to enjoy the Magellanic Clouds before they fade from sight. The results were presented this week at a meeting of the American Astronomical Society in Seattle, Washington.
