Effects of Minor Mergers on the Coalescence of a Supermassive Black Hole Binary

We studied the possibility that minor mergers can resolve the loss cone depletion problem, which is a difficulty that occurs in the coalescence process of a supermassive black hole (SMBH) binary, by performing numerical simulations with a highly accurate N-body code. We showed that a minor merger of a dwarf galaxy disturbs stellar orbits in the galactic central region of the host galaxy where loss cone depletion has already been caused by the SMBH binary. The disturbed stars are supplied into the loss cone. Stars of the dwarf galaxy are also supplied into the loss cone. The gravitational interactions between the SMBH binary and these stars become very effective. The gravitational interaction decreases the binding energy of the SMBH binary effectively. As a result, shrinking of the separation of the SMBH binary is accelerated. Our numerical results strongly suggest that minor mergers are one of the important processes to reduce the coalescence time of the SMBH binary to much less than the Hubble time.