The early hints that a small galaxy merged with the Milky Way are based on data gathered for 1500 stars as part of the Anglo-Australian Old Stellar Populations Survey (AAOSPS), carried out at the Anglo-Australian Observatory in New South Wales, Australia. Current theories of galaxy formation propose that many small galaxies came together to produce the Milky Way. But until now astronomers had found no evidence for the absorbed galaxies.

The team hopes that the study of a further 10 000 stars in the so-called thick disk of the Milky Way will strengthen its evidence. The thick disk is deeper than the well-known thin disk, which is the main feature of our galaxy. "This puffing up effect is most likely to be the result of the entry of a fairly massive satellite galaxy into the Milky Way", says Wyse.

It is thought that the thick disk formed as the Milky Way absorbed the orbital energy of the incoming galaxy. The creation of the thick disk is thought to be the last significant change in the Milky Way's structure. Wyse's earlier work suggests that the thick disk formed 10 billion years ago - when the Milky Way was only a third of its current age. This places important limitations on theories of galaxy formation.

Current cosmological theories suggest that around 70% of the matter in the universe is 'dark matter' - material that cannot be seen, but which influences the motion of stars and galaxies. According to this view, small galaxies cluster together to form large galaxies. But this model also predicts that many small galaxies would escape the process - leaving more small galaxies than astronomers currently observe. The discovery by Wyse and colleagues of a possible galactic remnant within the Milky Way will certainly add to the debate.