The results fit neatly with a growing body of evidence that stellar strangers abound in the Milky Way. Indeed, previous work has suggested that a number of globular clusters originated in other galaxies and were eventually absorbed by ours. But there has been no evidence that the most metal-poor clusters (and therefore supposedly the oldest) share that history. Suk-Jin Yoon and Young-Wook Lee of Seouls Yonsei University thus focused on these lowest metallicity aggregates, otherwise known as group II-b. With the help of computer models, the team used the colors of certain stars to determine the ages of clusters in this group and those in a group characterized by intermediate metal abundance. Surprisingly, the group II-b clusters turned out to in fact be younger than their more metal-rich counterparts.
Furthermore, the researchers found that most II-b clusters lie in the same plane as two of our galactic neighbors: the Large Magellanic Cloud (LMC) and the Draco dwarf galaxy. This unusual arrangement, along with hints that the II-b assemblages have an orbit similar to that of the LMC, suggests that they were forged in one of the nearby galaxies and only later acquired by the Milky Way. (Exactly when this celestial kidnapping might have occurred remains unknown.) Only more data will enable astronomers to pinpoint which of the Milky Ways resident globular clusters are natives and which are aliens. But for now, as Christine Clement of the University of Toronto says in an accompanying commentary, "evidence is mounting that the formation of our galaxy was less straightforward than [the monolithic model] suggests."