Andromeda feasts on its satellite galaxies.

Often referred to as the Milky Way's big sister, the nearby galaxy Andromeda is about twice as large as our own galaxy and has a similar spiral shape. A new study reveals another feature that the two have in common: Both are cannibals.

Observations of distant galaxies still forming reveal that they grow bigger by gravitationally capturing smaller galaxies that fall within their grasp. The captured material ends up in a spherical halo of matter surrounding the galaxy's disk. In our fully formed Milky Way, researchers have found several streams of gas and stars that appear to be the stretched-out remains of small galaxies gobbled up billions of years ago (SN: 4/22/00, p. 261).

Now, a giant stream of stars discovered in Andromeda's halo suggests that it also is devouring its neighbors, report Rodrigo Ibata of the Strasbourg Observatory in France, Michael Irwin of the University of Cambridge in England, and their colleagues in the July 6 NATURE.

"We suspected that Andromeda, like every other galaxy, formed by cannibalism," but there was no direct proof, comments Amina Helmi of the Max Planck Institute for Astrophysics in Garching, Germany. "This is why the stream is the smoking gun; it's the first hint that Andromeda actually formed by mergers of smaller galaxies."

Because star streams are both faint and stretched out, only a sensitive light detector that views a large area of sky can observe them. Even with such a device attached to the 2.5-meter Isaac Newton Telescope in the Canary Islands, Spain, Ibata's team required a week of observations to image the southern half of Andromeda's halo.

The stars and star streams found in the Milky Way's halo are deficient in elements heavier than helium, an indication that they originated from the metal-poor, first generation of stars in the cosmos. In contrast, Andromeda's halo and star stream have a higher abundance of heavy elements, which could only be made by later generations of stars, the researchers note.

The newly observed stream lies along a line connecting Andromeda to two of its satellite galaxies, M32 and NGC 205. Moreover, the stars in these satellites have an elemental composition similar to that of Andromeda's halo. This suggests that Andromeda has recently captured stars from one or both of these bodies, the team says. The astronomers would expect, in accordance with observations of star streams in the Milky Way, to find a similar star stream on the northern side of Andromeda's halo.

"The discovery of the Andromeda stream in the first deep, panoramic survey of the Milky Way's nearest large companion suggests that halo substructures in the form of giant tidal streams may be a generic property of large spiral galaxies and that the formation of galaxies continues at a moderate pace up to the present day," Ibata and his colleagues note.

Such star streams may have a bearing on one of astronomy's most tantalizing puzzles. Evidence suggests that dark matter, the mystery material proposed to make up more than 90 percent of the mass of the universe, is concentrated in the halos of galaxies. The star stream detected by Ibata's team may help trace the distribution of this dark matter in Andromeda, Helmi says.