Effects of Minor Mergers on Coalescence of a Supermassive Black Hole   Binary
  We study the possibility that minor mergers resolve the loss cone depletion problem, which is the difficulty occured in the coalescence process of the supermassive black hole (SMBH) binary, by performing numerical simulations with a highly accurate $N$-body code. We show that the minor merger of a dwarf galaxy disturbs stellar orbits in the galactic central region of the host galaxy where the loss cone depletion is already 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, the shrink of the separation of the SMBH binary is accelerated. Our numerical results strongly suggest that the minor mergers are one of the important processes to reduce the coalescence time of the SMBH binary much less than the Hubble time. 
Hidenori Matsui and Asao Habe