Gravitational Interactions of Binary and Triple Black Holes

In the context of a hierarchical structure formation scenario describing the evolution of a Super Massive Black Holes (SMBHs) population from primordial seeds, remnants of the first POPIII stars, gravitational interactions between two or three black holes play an intersting role. If the formation route for the assembly of SMBHs dates back to the early universe, a large number of BH interactions is inevitable. If most galaxies have a central BH and the BH inspiral time is less than a Hubble time, binary black holes are inevitably formed following galactic mergers. Binary black holes coalescence timescales are long enough for a third BH to fall in and interact with the central binary. These BH slingshots modify the binary binding energy, but lead also to the final expulsion of one of the three bodies and to the recoil of the binary. Also, asymmetric emission of gravitational waves in the last stages of the black hole merging can give a recoil velocity to the center of mass of the coalescing binary (gravitational rocket). We show how slingshots can help binaries to merge over shorter timescales and how slingshots and rockets contribute creating populations of BHs outside galactic nuclei and halos.