Galaxies are large, but many of them—including the Milky Way—are much larger than suspected.
Astronomers at the Keck Observatory in Hawaii have found that the Andromeda galaxy—our closest galactic neighbor, 2.2 million light-years away
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Courtesy Robert Gendler
Our closest neighbor, The Andromeda galaxy, also known as M31, is visible to the naked eye. This image is a mosiaic of 20 frames taken with small telescope.
and a familiar sight in the evening sky—is three times bigger than previously thought. Spectrographic analysis shows that 3,000 stars, once believed to be separate from Andromeda, move in lockstep with the galaxy’s rotation, as part of its outer disk. “It was completely unexpected,” says Caltech astrophysicist Scott Chapman, who made the discovery with Rodrigo Ibata of the Astronomical Observatory in Strasbourg, France.
In a related finding, new images from the Gemini South telescope in Chile exposed another giant disk around NGC 300, a far less massive galaxy that resembles our own but is 6 million light-years away. Using Gemini’s high-end optics, Joss Bland-Hawthorn of the Anglo-Australian Observatory and astronomer Ken Freeman of Australian National University counted the brightest stars on the galaxy’s fringes. The census shows that, like Andromeda, NGC 300 boasts a broad disk—in this case one that effectively doubles the galaxy’s size.
The new measurements mean that astronomers must rethink how galaxies formed. “The one thing that is clear is that all the models of galaxy formation do not predict such large spinning disks,” says Chapman. These models typically assume that galaxies were created by gases clumped together by gravity. Since the gases thin out away from the core, star concentration should ease rapidly, and galaxies should exhibit sharp edges. But the stars appear to be obeying unknown rules, tapering off evenly. Princeton University astrophysicist Bruce Draine, who has worked on the theoretical side of the Gemini project, says the findings may offer surprises about the Milky Way. “It is reasonable to presume that stars formed in our Milky Way outside the disk and that the disk might well extend out to very large distances.” —Alex Stone