Never two without three: binary black hole mergers via three-body encounters in young, globular, and nuclear star clusters

Since the first detection of gravitational waves in 2015, the population of compact binary objects has grown year after year, counting now more than 60 confirmed binary black hole (BBH) mergers. With such a rapidly increasing population, more and more questions are arising. The detection of massive black holes (BH) in the 60-120 M_⊙ mass range, the spin discrepancy with the BH population in X-ray binaries, and the observation of possible processing BBHs are only a few of these open issues that urgently seek an answer. In my talk, I will show how three-body encounters between BHs may represent a way out to solve this BH population puzzle. Three-body interactions are the most frequent kind of encounters that can take place between BHs inside crowded stellar environments. Starting from the results of my N-body simulations, I will present how these interactions differently affect the BH population of young, globular, and nuclear star clusters and what is their role in the formation of BBH mergers. The mass, spin, and eccentricity of the BH binaries are strongly influenced by these interactions and by the hosting environment. Different properties may therefore be used as fingerprints to derive the birthplace of these mergers from the observations, and to solve these still-open issues.