A new mechanism for massive binary black hole coalescences

Massive binary black holes (BBHs) are inevitably formed in the merged galactic nuclei before the black holes finally merge by emitting the gravitational radiation. However, it is still unknown how a massive BBH evolves after its semi-major axis reached to the sub-parsec/parsec scale where the dynamical friction with the neighboring stars is ineffective (the so-called the final parsec problem). We find a new mechanism by which massive BBHs can naturally coalesce within a Hubble time. In this paper, we study the evolution of the BBH with triple disks which are composed of an accretion disk around each black hole and one circumbinary disk surrounding them. While the circumbinary disk removes the orbital angular momentum of the BBH via the binary-disk resonant interaction, the mass transfer from the circumbinary disk to each black hole adds some fraction of its angular momentum to the orbital angular momentum of the BBH. We find that there is a critical value of the mass-transfer rate where the extraction of the orbital angular momentum from the BBH is balanced with the addition of the orbital angular momentum to the BBH. The semi-major axis of the BBH decays with time whereas the orbital eccentricity of the BBH grows with time, if the mass transfer rate is smaller than the critical one, and vice versa. These timescales are characterized by the product of the viscous timescale of the circumbinary disk and the ratio of the total black hole mass to the mass of the circumbinary disk. Since a minimum value of the critical mass-transfer rate is larger than the Eddington accretion rate of massive black holes with masses in the 10M⊙ to 10 M⊙ range, it is promising that the critical mass-transfer rate is larger than the mass transfer rate. Most of massive BBHs, therefore, enable to merge within a Hubble time by the proposed mechanism, which helps to solve the final parsec problem.