A giant stream of metal-rich stars in the halo of the galaxy M31

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Abstract
The Milky Way is a cannibal. Recent observations have revealed the detritus of its digestion of the Sagittarius Dwarf and Magellanic Clouds, apparent as streams of gaseous and stellar debris littering the Galactic Halo. The analysis of this material has shown that the dark matter distribution within our own Galaxy is nearly spherical, and that much of the Galactic Halo is made up of cannibalized satellite galaxies. Yet it remains unclear whether such halo substructures really are as ubiquitous and as numerous as predicted by galaxy formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the Andromeda galaxy (M31), the Milky Way's nearest large neighbour. The source of this stream could be the dwarf galaxies M32 and NGC 205, close companions of M31, which have possibly lost a substantial amount of stars due to their tidal interactions with their large neighbour. These observations clearly demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that substructure in the form of huge, recently-deposited tidal streams, may be a generic feature of galaxy halos.

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