Like a child leaving a trail of crumbs when emptying a cookie jar, the Andromeda galaxy contains a stream of stars that suggests it is eating its smaller companion galaxies. Astronomers aren't sure, however, whether the crumbs came from devoured satellite galaxies or surviving companions.
Similar streams of stars--curved, elongated regions with high stellar densities--have been found in the outer halo of our own Milky Way galaxy. They form when a large galaxy's gravity pulls one edge of a nearby satellite galaxy more strongly than the other edge, unraveling the galaxy and leaving stars behind.
Suspecting that Andromeda, a nearby giant spiral galaxy similar to the Milky Way, might sport a similar stream, researchers led by Rodrigo Ibata of the Strasbourg Observatory in France used the Isaac Newton Telescope at La Palma, Canary Islands, to find out. Sure enough, they found a giant stream measuring at least 120,000 light-years long. The stars might have been pulled from one or both of Andromeda's current companion galaxies, or they might be the remains of a third companion, which has fully dispersed, the team reports in the 5 July issue of Nature.
According to Tim de Zeeuw of Leiden University in the Netherlands, Andromeda's two companions were already known to be slightly misshapen. Piet van der Kruit of Groningen University in the Netherlands adds that it is still unclear how important cannibalism has been for the evolution of galaxies like our Milky Way or Andromeda. "The situation is much more complicated than we thought a couple of years ago," he says.
--GOVERT SCHILLING
Related sites
The Isaac Newton Telescope
The Andromeda galaxy
