Star cluster properties from intermediate-mass black hole mergers

It is believed that intermediate-mass black holes (IMBHs) with masses in the range from 100 to one million solar masses provide the seeds of supermassive black holes. However, IMBHs have a small radius of influence, and they are difficult to identify electromagnetically. A fraction of these IMBHs will merge with each other or with smaller black holes generating gravitational waves that can be detectable with future gravitational-wave observatories, such as the Einstein Telescope, Cosmic Explorer, or LISA. Successive black hole mergers and repeated tidal disruption events can lead to the formation of a massive black hole runaway in the cores of dense star clusters. We discuss how single IMBH-IMBH events — occurring after the merger of clusters that sank into the center of their host galaxies — can be used to infer the properties of their progenitor star clusters.