The Potential for Long-lived Intermediate-mass Black Hole Binaries in the Lowest Density Dwarf Galaxies

Intermediate-mass black hole (IMBH) mergers with masses 10⁴-10⁶ M _⊙ are expected to produce gravitational waves detectable by the Laser Interferometer Space Antenna (LISA) with high signal-to-noise ratios from the present day to cosmic dawn. IMBH mergers are expected to take place within dwarf galaxies; however, the dynamics, timescales, and effect on their hosts are largely unexplored. In a previous study, we examined how IMBHs would pair and merge within nucleated dwarf galaxies. IMBHs in nucleated hosts evolve very efficiently, forming a binary system and coalescing within a few hundred million years. Although the fraction of dwarf galaxies (10⁷ M _⊙ ≤ M _⋆ ≤ 10¹⁰ M _⊙) hosting nuclear star clusters is between 60% and 100%, this fraction drops to 20%-70% for lower-mass dwarfs (M _⋆ ≈ 10⁷ M _⊙), with the largest drop in low-density environments. Here, we extend our previous study by performing direct N-body simulations to explore the dynamics and evolution of IMBHs within nonnucleated dwarf galaxies, under the assumption that IMBHs exist within these dwarfs. To our surprise, none of the IMBHs in our simulation suite merge within a Hubble time, despite many attaining high eccentricities e ∼ 0.7-0.95. We conclude that extremely low stellar density environments in the centers of nonnucleated dwarfs do not provide an ample supply of stars to interact with an IMBH binary, resulting in its stalling, in spite of triaxiality and high eccentricity, common means to drive a binary to coalescence. Our findings underline the importance of considering all detailed host properties to predict IMBH merger rates for LISA.