Dynamics of binary black holes in young star clusters: the impact of cluster mass and long-term evolution

Dynamical interactions in dense star clusters are considered one of the most effective formation channels of binary black holes (BBHs). Here, we present direct N-body simulations of two different star cluster families: low-mass (similar to 500-800 M-circle dot) and relatively high-mass star clusters (>= 5000 M-circle dot). We show that the formation channels of BBHs in low- and high-mass star clusters are extremely different and lead to two completely distinct populations of BBH mergers. Low-mass clusters host mainly low-mass BBHs born from binary evolution, while BBHs in high-mass clusters are relatively massive (chirp mass up to similar to 100 M-circle dot) and driven by dynamical exchanges. Tidal disruption dramatically quenches the formation and dynamical evolution of BBHs in low-mass clusters on a very short time-scale (less than or similar to 100 Myr), while BBHs in high-mass clusters undergo effective dynamical hardening until the end of our simulations (1.5 Gyr). In high-mass clusters, we find that 8 per cent of BBHs have primary mass in the pair-instability mass gap at metallicity Z = 0.002, all of them born via stellar collisions, while only one BBH with primary mass in the mass gap forms in low-mass clusters. These differences are crucial for the interpretation of the formation channels of gravitational-wave sources.

Automated summary

Dynamical interactions in dense star clusters are effective formation channels for binary black holes. The formation processes of low-mass and high-mass star clusters lead to two distinct populations of binary black hole mergers. Tidal disruption hinders the formation and evolution of binary black holes in low-mass clusters, while high-mass clusters undergo effective dynamical hardening. These differences are crucial for understanding the formation channels of gravitational-wave sources.