2020NatAs...4..260J

The direct measurement of gravitational waves is a powerful tool for surveying the population of black holes across the universe. While LIGO has detected black holes as heavy as ∼50 M☉ (ref. 1), there is not yet unambiguous evidence of black holes in the intermediate mass range of 102-5 M☉. Recent electromagnetic observations have hinted towards the existence of intermediate-mass black holes (IMBHs)2-4; however, their masses remain poorly constrained. Here we argue that multiband observations by space- and ground-based gravitational wave detectors5,6 will be able to survey a broad population of IMBHs at cosmological distances. By utilizing general relativistic simulations of merging black holes7 and state-of-the-art gravitational waveform models8, we classify three distinct populations of binaries with IMBHs and discuss what can be observed about each. Multiband observations involving the upgraded LIGO detector9 and the proposed space-mission LISA10 would detect the inspiral, merger and ring-down of IMBH binaries out to redshift ≃ 2, pushing out to redshift ≃ 5 if next-generation ground-based detectors11,12 are operational at the same time. To facilitate studies of multiband IMBH sources, we provide analytic relations for the maximum redshift of multiband detectability, as a function of black hole mass, for various detector combinations. Our study paves the way for future work on what can be learned from IMBH observations in the era of multiband gravitational wave astronomy.