The Andromeda galaxy isn't as restrained and helpless as its mythological namesake. The large spiral galaxy has been consuming stars from some of its neighboring dwarf galaxies, scientists say.
A team led by Rodrigo Ibata of France's Strasbourg Observatory discovered a stream of stars on Andromeda's outer reaches that appear to have been stripped from the dwarf galaxies M32 and NGC 205.
"It shows that small galaxies are being eaten up by big galaxies," Ibata told Reuters. The findings were published in this week's issue of the journal Nature.
"Large galaxies have been formed by the merging of many smaller galaxies in a hierarchical system. The smaller systems are the first able to be formed, then they start interacting forming larger structures." The finding supports the ideas that big galaxies have built up over time through the collisions of smaller galaxies colliding and that the process continues today.
Andromeda, our nearest large galactic neighbor, is our Milky Way galaxy's "big sister," twice as large but otherwise very similar. It is 2.2 million light-years away from the Milky Way.
Like the Milky Way, the spiral structure of Andromeda is surrounded by a sphere or halo of stars and unseen "dark matter," Ibata said. The group of stars discovered is in Andromeda's halo.
Astronomers have known for some years that our galaxy is a cannibal. Its outer parts are threaded through with telltale streams of stars from small galaxies it has engulfed.
Like a rat swallowed by a snake, stars of a swallowed galaxy stay recognizable as a lump for billions of years, eventually mingling within the stars of their conqueror.
"This has given the outskirts of our galaxy, its 'halo', a rather lumpy structure," said Ibata. "We wanted to see if Andromeda's halo was the same."
The new survey was possible only because the digital devices that have replaced photography in astronomy have now been developed enough to cover fairly large areas of sky. Even so, more than 50 long exposures had to be pieced together to give a panorama of the halo on only one side of Andromeda.
The ripped-off stars can be seen as a distinct stream. They can also be distinguished from other stars in Andromeda's halo by their slightly different chemical composition.
"Andromeda is close enough for us to be able to see individual stars stripped from the satellite galaxies and measure their speeds. From this we will be able to map the distribution of 'dark matter' in the halo surrounding Andromeda," said team member Geraint Lewis. "No-one has been able to do this definitively before."
In the ultimate corporate merger, our galaxy is predicted to collide with Andromeda three billion years from now. Astronomers have known for almost a century that the two galaxies are falling together at a little more that 300,000 mph (500 000 kilometers an hour).
"Colliding with a dwarf galaxy is only like having a cream pie hit your windscreen," Lewis said. "When our galaxy and Andromeda collide it'll look like a car crash -- very messy."
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