科学美国人60秒:Galaxy Gave Star Cluster the Boot

If galaxies were high school lunchrooms, the star cluster named HVGC-1 would be a social pariah. This unpopular cluster has been kicked out of its galaxy and forced to wander the cosmos alone.

HVGC-1 stands for hypervelocity globular cluster 1. Its thousands of stars had been part of the M87 galaxy, about 50 million light-years away. But now the cluster is fleeing that galaxy at more than two million miles per hour. Researchers reported the discovery in The Astrophysical Journal Letters. [Nelson Caldwell et al, A Globular Cluster Toward M87 with a Radial Velocity < -1000 km/s: The First Hypervelocity Cluster]

Astronomers are not sure why HVGC-1 was exiled from M87. One theory suggests that gravitational interactions with a pair of supermassive black holes at the galaxy’s center could have kicked out the cluster. Most galaxies contain a single giant black hole. If M87 is the product of a merger of two galaxies, it might host two central black holes in a binary system, researchers say.
 
In fact, that same double black hole future might await the Milky Way when we collide with our neighboring galaxy, Andromeda, in a few billion years. At which point, some star cluster may find itself ejected from the Milky Way, just like the ostracized HVGC-1.
 
—Clara Moskowitz