MINNEAPOLIS, MINNESOTA--The Andromeda galaxy, our heavenly neighbor, is even grander than astronomers realized. A new survey of Andromeda's outskirts reveals that its graceful disk of stars extends 3 times farther into space than previous images had suggested. How that vast disk arose is a mystery, according to a report here 30 May at a meeting of the American Astronomical Society.
Andromeda is visible in the northern hemisphere as a faint fuzzy patch, about 2 million light-years away. A near-twin of our Milky Way, Andromeda contains at least 100 billion stars in a flattened disk of spiral arms, calmly orbiting a round bulge of stars in the center. Although the disk appeared to span less than 100,000 light-years, astronomers had seen sprinkles of other stars scattered far beyond the disk at the same distance from Earth, suggesting that the stars also belonged to the galaxy. But these outliers seemed to wander around Andromeda on random paths, like moths around a streetlamp.
To take a better galactic census, a team led by astronomer Rodrigo Ibata of the Strasbourg Observatory in France took the most detailed images yet of the space around Andromeda, exposing swarms of faint stars distributed near the galaxy. Then, astronomer Scott Chapman of the California Institute of Technology (Caltech) in Pasadena and his colleagues used the Keck II Telescope in Hawaii to track the orbits of about 5000 of those stars by gauging their motions toward or away from Earth. When the team plotted the results, they were stunned to find that most of the distant stars rotate as an extension of Andromeda's disk, in an orderly pattern. The far-flung stars are a significant part of Andromeda, Chapman notes; they may produce 10% of the galaxy's total light.
The vast new component of the galaxy is perhaps 300,000 light-years across--as wide as 12 full moons in the sky, if we could see the full disk with our eyes. Clumps within the extended disk show that Andromeda has absorbed stars in batches, Chapman says. "It clearly formed by lots of little galaxies crashing into Andromeda over time," he says. But that scenario presents a puzzle: Models predict that when the gravity of a giant galaxy captures lots of smaller ones, the stars should disperse into random orbits.
The results are convincing but hard to understand, says astronomer Wallace Sargent of Caltech, who was not part of the study. "I find it very impressive, but I didn't expect to see ordered motion so far out," he says. Some unknown influence must settle the orbits and make them persist for billions of years, Sargent says.