Two Ancient Migration Events in the Andromeda Galaxy

The globular clusters studied (lower right insets), indicated by colored circles, are located in the outer halo of the Andromeda Galaxy, beyond the bright disk of the galaxy (upper left inset). The star clusters separate into two groups — those associated and unassociated with stellar streams — that have very different orbits, a result that points to two discrete migration events in the history of the galaxy. The color of each circle indicates the line-of-sight velocity of the corresponding star cluster.