If the Milky Way did grow by swallowing up smaller galaxies, then another team suspects it knows where in the Milky Way some of those alien stars are hiding.
Duncan Forbes of Swinburne University of Technology and his Canadian colleague Terry Bridges are using Hubble Space Telescope data to identify clusters of alien stars, using the fact that their age and chemical composition differs from their neighbours.
Globular clusters are dense spherical collections of about a million stars of the same age. Held tightly together by gravity, they move together as a unit. Based on the Hubble data, Duncan and Terry think that, over the past few billion years, about a quarter of the globular star clusters in our galaxy—tens of millions of stars—formed elsewhere before moving into the Milky Way.
The two researchers think that many of the alien clusters may initially have formed in mini- or dwarf galaxies of about 100 million stars that were later swallowed up by the Milky Way. And when the larger galaxy structure was broken down, the clusters remained intact.
“Astronomers have already confirmed the existence of two accreted dwarf galaxies in our Milky Way—but our research suggests that there might be as many as six yet to be discovered,” Duncan says. “This will have to be explored further, but it is a very exciting prospect that will help us to better understand the history of our own galaxy.”
The work was carried out in Canada as part of an Australian Research Council International Fellowship.