How do binary black-hole systems form?
Back in September 2015, astronomers using the incredibly complex Laser Interferometer Gravitational-Wave Observatory (LIGO), detected for the first time gravitational waves resulting from the merger of two black holes. Three months later (on Boxing Day), LIGO caught another pair of black holes in the act of merging. This second detection suggests that the first was no one-off fluke. Are black hole pairs really that common?
According to Fred Rasio (Northwestern University), astrophysicists think there are two ways to form black hole binaries. In one theory, two massive stars already in a binary system evolve and die, usually as supernovae. If the binary survives as each star collapses into a black hole—not a sure thing since supernova explosions are very violent—then a black hole pair is the result.
The second—a dynamical-formation scenario—was discussed by Rasio at the June 2016 American Astronomical meeting. It assumes that in very dense star clusters such as globular clusters, stellar-size massive black holes already exist, born via normal stellar evolution,
“In such dense systems, stars interact dynamically all the time,” he explained. “Eventually the black holes settle down and go where gravity takes them, which in these systems is the centre of the cluster where the density is highest and the dynamical interactions are most frequent and most violent. This is what my graduate student called the black hole mosh pit.”
The mosh pit at the heart of a cluster is typically only a few light-years across, but contains thousands of stars. Here black holes interact, form pairs, and even swap partners before pairs are eventually ejected from the cluster—all due to dynamical interactions with other black holes. Rasio’s computer simulations indicate that these ejected black hole binaries are “just the kind of system that will eventually become a LIGO source when the two black holes merge.”