Extremely efficient mergers of intermediate-mass black hole binaries in nucleated dwarf galaxies

Gravitational waves emitted by merging black holes between similar to 10(4) and 10(7) M-circle dot will be detectable by the Laser Interferometer Space Antenna (LISA) with signal-to-noise ratios of several hundred out to redshift 20. Supermassive black hole (10(7)-10(10) M-circle dot) binary formation, coalescence, and merger within massive galaxies is well-studied. However, intermediate-mass black holes (IMBHs) between similar to 10(4) and 10(6) M-circle dot are hosted by low mass and dwarf galaxies; it is not trivial to extrapolate black hole merger time-scales to this IMBH binary regime, due to the starkly different host galaxy structure, kinematics, and morphology compared to massive galaxy hosts. We perform ultrahigh resolution N-body simulations to study IMBH dynamics in nucleated dwarf galaxies whose structural parameters are obtained from observations of nearby dwarf galaxies. Starting from 50 pc, an IMBH quickly forms a binary. Thereafter, the binary orbit shrinks rapidly due to the high central stellar densities furnished by nuclear star clusters (NSCs). We find high eccentricities (e similar to 0.4-0.99) in our suite of IMBH binaries, and residual eccentricity may persist to the LISA regime. IMBH merger times are typically a few hundred million years, with a few exceptionally short merger times for high eccentricities. We find that IMBH-stellar encounters originate pre-dominantly from NSCs, if the NSC-to-IMBH binary mass ratio is greater than 10; otherwise, bulge stars contribute significantly. As the IMBH binary ejects stars, however, the NSCs are disrupted. We conclude that comparable-mass IMBHs merge very efficiently in nucleated dwarf galaxies, making them promising LISA sources, as well as a channel for IMBH growth.

Automated summary

Gravitational waves emitted by merging black holes with masses between approximately 10(4) and 10(7) solar masses can be detected by LISA with high signal-to-noise ratios. Studying intermediate-mass black holes (IMBHs) in dwarf galaxies, the research found that IMBHs in these environments form binaries quickly and efficiently merge due to interactions with nuclear star clusters. The study suggests that IMBHs in nucleated dwarf galaxies are promising sources for LISA observations and could be significant for the growth of IMBHs.