A binary black hole can form via close encounters of black holes in a dense stellar environment, such as a nuclear star cluster, a globular cluster or a young star cluster. In this case, the two black holes may be single objects at birth, and pair up dynamically at some point in their “life”. When two stellar-born black holes merge via gravitational wave emission, their merger remnant is called second-generation (2g) black hole. The 2g black hole is a single object at birth. However, if it is retained inside its host star cluster, it may pair up dynamically with another black hole. This gives birth to what we call a second-generation (2g) binary black hole, i.e. a binary black hole that hosts a 2g black hole . If a 2g binary black hole merges again, it gives birth to a third- generation (3g) black holes, and so on. In this way, repeated black hole mergers in star clusters can give birth to hierarchical chains of mergers, leading to the formation of more and more massive black holes.
Understand the differences between hierarchical binary black hole mergers in nuclear star
clusters, globular clusters and young star clusters, by looking at a set of simulated binary black
holes. Nuclear star clusters are very massive (~1e5 – 1e8 solar masses) star clusters lying at the
center of some galaxies, including the Milky Way. Globular clusters are old (12 Gyr) massive (
1e4 – 1e6) stellar clusters lying in the halo of almost every galaxy. Young star clusters are young
(<100 Myr) stellar clusters forming mostly in the disk of a galaxy.