Formation of massive black holes in ultracompact dwarf galaxies: migration of primordial intermediate-mass black holes in N-body simulation

Recent observational studies of ultracompact dwarf galaxies (UCDs) have discovered the massive black hole (MBH), with masses of more than 10(6) M-circle dot, in their central regions. We here consider that these MBHs can be formed through the merging of intermediate-mass black hole (IMBH), with masses of (10(3)-10(5)) M-circle dot, within the stellar nuclei of dwarf galaxies, which are progenitors of UCDs. We numerically investigate this formation process for a wide range of model parameters using N-body simulations. This means that IMBH growth and feedback is neglected in this study. We find that only massive IMBHs of 10(5) M-circle dot sink into the central regions of their host dwarf (approximate to 10(10) M-circle dot) to be gravitationally trapped by its stellar nucleus within less than 1 Gyr in most dwarf models. We also find that lighter IMBHs with (1-30) x 10(3) M-circle dot sink into the centre in low-mass dwarfs 10(9) M-circle dot) due to more efficient dynamical frictionitionally, we show that the IMBHs can form binaries in the centre and, rarely, before they reach the centre, which may lead to the IMBHs merging and thus emitting gravitational waves that could be detected by LISA. Finally, we discuss the required number of IMBHs for the MBH formation in UCDs and the physical roles of stellar nuclei in IMBH binaries and mergers.