Alien Star Clusters in the Milky Way
Canadian astronomer Terry Bridges explains how he and a colleague tracked down star clusters in the Milky Way that were stolen from other galaxies
By Terry Bridges
When my colleague Duncan Forbes, from Swinburne University in Melbourne, Australia, visited Queen’s University, Kingston, Ontario, last summer, we decided to work on a project involving globular star clusters. These beautiful objects, containing anywhere from 10,000 to a million stars, are fascinating in their own right. But their high luminosities (sometimes more than 100,000 times the brightness of the Sun) and old ages (10 to 14 billion years) also give us information about their host galaxies. For instance, Duncan and I have been using globular clusters to study the ages, chemical compositions and dark-matter content of nearby elliptical galaxies. For this project, however, we wanted to understand the origin of globular clusters in our own galaxy, the Milky Way.
A typical spiral galaxy, the Milky Way has about 150 globular clusters, and we think that most of these were formed when the galaxy was young, some 13 billion years ago. However, there is good evidence that some of the Milky Way’s clusters were stolen from other galaxies. The Milky Way is currently accreting two dwarf galaxies known as Sagittarius and Canis Major (discovered in 1994 and 2004, respectively, by astronomer Rodrigo Ibata and his collaborators at France’s Observatoire Astromique de Strasbourg). Dwarf galaxies are much smaller than the Milky Way and are the only type near enough to be captured by our galaxy. Interestingly, a handful of globular clusters that were originally associated with Sagittarius and Canis Major are now part of our galaxy’s globular-cluster system. We asked ourselves: How many more captured globular clusters are there in the Milky Way and how can we identify them?
Duncan and I thought that these “alien invaders” might have different ages and chemical compositions than the Milky Way’s native clusters. So we scoured the literature and found 93 clusters with reliable measurements of age and composition. When we plotted cluster composition versus age, we saw two distinct populations of clusters: an old population (12 to 14 billion years old) with a range of compositions and a population extending to younger ages (6 to 11 billion years old). We were excited to see that the globular clusters known to be associated with Sagittarius and Canis Major were mainly part of the younger population, and we were able to distinguish between captured and native globular clusters on the basis of their ages and compositions.
After the probable members of Sagittarius and Canis Major are accounted for, there are still several clusters with younger ages that could have been captured from other dwarf galaxies. Further clues came from looking at the orbits and locations on the sky of these “capture candidates.” Sixteen of the Milky Way’s clusters have retrograde orbits (i.e., they orbit around the Milky Way in the opposite direction to most clusters). These are prime candidates for having been captured from a dwarf galaxy, just as moons of Jupiter and Saturn with retrograde orbits are thought to be captured asteroids. Approximately half of the retrograde clusters are in the young population and are thus likely captured objects. The second clue is that several of the dwarf galaxies orbiting the Milky Way are found on a great circle on the sky. Of the 10 globular clusters on this great circle, seven are in the young population. We speculate that these clusters were originally associated with a dwarf galaxy that has since been completely disrupted.
Summing up, we estimate that between 27 and 47 of the Milky Way’s globular clusters were captured from six to eight dwarf galaxies. In other words, perhaps one-quarter of the Milky Way’s globular clusters were not born here. In fact, there are likely more invaders to be found, as there are still many Milky Way globular clusters without measured ages, chemical compositions and orbits. More data, together with detailed studies of the candidate captured clusters, are needed to obtain firmer estimates. Our work is a step toward a greater understanding of how galaxies like the Milky Way are assembled through the capture and destruction of smaller galaxies. Our study was published in Monthly Notices of the Royal Astronomical Society, in March of this year.