We present the results of N-body simulations of dynamical evolution of triple massive black hole (BH) systems in galactic nuclei. We found that in most cases two of the three BHs merge through gravitational wave (GW) radiation in a timescale much shorter than the Hubble time before ejecting one BH through a slingshot. In order for a binary BH to merge before ejecting the third one, it has to become highly eccentric, since the GW timescale would be much longer than the Hubble time unless the eccentricity is very high. We found that two mechanisms drive the increase of the eccentricity of the binary. One is the strong binary-single BH interaction, which results in the thermalization of the eccentricity. The second is the Kozai mechanism, which drives the cyclic change of the inclination and eccentricity of the inner binary of a stable hierarchical triple system. Our result implies that many supermassive BHs are binaries.
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