Our neighboring Andromeda Galaxy may look benign, but it's really a massive cannibal, sitting surrounded by the undigested remnants of the galaxies it's eaten. European scientists have mapped the galaxy anew, and found stars that came from elsewhere.
Writing in the new issue of Nature, the researchers explain that Andromeda is surrounded by a halo of remnants from other galaxies, and this provides evidence for the "hierarchical" model of galaxy development, where bigger galaxies grow by eating smaller ones:
In hierarchical cosmological models1, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk2, 3. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history4, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.
And here's a cool looking picture of Andromeda's orbit that they released:
Image of Triangulum Galaxy from NASA.