Intermediate Mass Ratio Inspirals in Milky Way Galaxies

Evidence for intermediate mass black holes (IMBHs) in dwarf galaxies is strong, and a consequence of a non-zero occupation fraction of IMBHs in dwarfs is that these IMBHs can become residents of larger galaxy halos via hierarchical merging and tidal stripping. Depending on the parameters of their orbits and original hosts, some of these IMBHs will merge with the central supermassive black hole in the larger galaxy. We examine four cosmological zoom-in simulations of Milky Way-like galaxies to study the demographics of the black hole mergers which originate from dwarf galaxies. Approximately half of these mergers have mass ratios less than 0.04, which categorizes them as intermediate mass ratio inspirals, or IMRIs. Inspiral durations range from 0.5 - 8 Gyr, depending on the initial inclination of the orbit, mass ratio, and compactness of the dwarf galaxy. The eccentricity of the inspiral often becomes more circular with time, resulting in 0 < e < 0.1 mergers, but this is not always the case. Overall, IMRIs from IMBHs in Milky Way-like galaxies are a significant class of black hole merger that can be detected by LISA, and must be prioritized for waveform modeling.