Detectability of Intermediate-Mass Black Holes in Multiband Gravitational Wave Astronomy

The direct measurement of gravitational waves is a powerful tool for surveying the population of black holes across the universe. The first gravitational wave catalog from LIGO has detected black holes as heavy as $\sim50~M_\odot$, colliding when our Universe was about half its current age. However, there is yet no unambiguous evidence of black holes in the intermediate-mass range of $10^{2-5}~M_\odot$. Recent electromagnetic observations have hinted at the existence of IMBHs in the local universe; however, their masses are poorly constrained. The likely formation mechanisms of IMBHs are also not understood. Here we make the case that multiband gravitational wave astronomy --specifically, joint observations by space- and ground-based gravitational wave detectors-- will be able to survey a broad population of IMBHs at cosmological distances. By utilizing general relativistic simulations of merging black holes and state-of-the-art gravitational waveform models, we classify three distinct population of binaries with IMBHs in the multiband era and discuss what can be observed about each. Our studies show that multiband observations involving the upgraded LIGO detector and the proposed space-mission LISA would detect the inspiral, merger and ringdown of IMBH binaries out to redshift ~2. Assuming that next-generation detectors, Einstein Telescope, and Cosmic Explorer, are operational during LISA's mission lifetime, we should have multiband detections of IMBH binaries out to redshift ~5. To facilitate studies on multiband IMBH sources, here we investigate the multiband detectability of IMBH binaries. 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.