Speaker
Description
Stellar clusters are efficient factories of dynamical interactions and play a crucial role in shaping the black hole (BH) mass distribution observed by gravitational-wave (GW) detectors. The ability of these dynamically active environments to pair and retain BHs enables the formation of remnants more massive than those typically produced through isolated binary evolution. This characteristic provides one of the key discriminants among different GW formation channels, as highlighted by recent events such as GW231123, possibly the first detected merger involving an intermediate-mass black hole (IMBH; mass above 100 solar masses).
In this work, we investigate the hierarchical assembly of binary BHs, with a specific focus on the formation and evolution of secondary branches within BH merger trees. We introduce a new formalism implemented in our semi-analytical population-synthesis code BPOP, which allows us to track the growth of structured hierarchical chains. These chains are a key channel for the formation of IMBHs, as hierarchical secondary mergers, while rare, preferentially populate the high-mass end of the remnant distribution. Furthermore, they leave signatures in the spin distribution, as their merger products are expected to have high spins.
In my talk, I will present the main results of this work and the expected rates for current and next-generation detectors such as ET. I will show that tracking the full hierarchical structure of secondary mergers is crucial to interpret the origin of the most massive black holes observed in GW events, especially in the high-mass tail of the distribution.