Observing IMBH-IMBH Binary Coalescences via Gravitational Radiation

Recent numerical simulations have suggested the possibility of forming double intermediate-mass black holes (IMBHs) via the collisional runaway scenario in young dense star clusters. The two IMBHs that formed would exchange into a common binary shortly after their birth and quickly inspiral and merge. Since space-borne gravitational wave (GW) observatories such as LISA will be able to see the late phases of their inspiral out to several gigaparsecs, and LIGO will be able to see the merger and ringdown out to similar distances, they represent potentially significant GW sources. In this Letter we estimate the rate at which LISA and LIGO will see their inspiral and merger in young star clusters, and we discuss the information that can be extracted from the observations. We find that LISA will likely see tens of IMBH-IMBH inspirals per year, while advanced LIGO could see 610 merger and ringdown events per year, with both rates strongly dependent on the distribution of cluster masses and densities.