2020MNRAS.496..921W

Recent observational studies of ultracompact dwarf galaxies (UCDs) have discovered the massive black hole (MBH), with masses of more than 10^6~M_☉, 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 (10³-10⁵) M_☉, 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_☉ sink into the central regions of their host dwarf (~10¹⁰∼ M_☉) to be gravitationally trapped by its stellar nucleus within less than 1 Gyr in most dwarf models. We also find that lighter IMBHs with (1g-30) ×10^3~M_☉ sink into the centre in low-mass dwarfs (~10⁹∼ M_☉) 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.