Forecasting Properties of Detectable Massive Binary Black Hole Mergers in the Era of Space-based Gravitational-wave Detectors

Abstract Gravitational waves (GWs) from massive black hole (MBH) mergers will provide a novel way to probe the high-redshift Universe and are key to understanding galactic dynamics and evolution. In this work, we analyze MBH mergers, their GW signals and detectability, as well as their population properties, using the cosmological hydrodynamical simulation NINJA Simulation Suite. We discuss the effect of resolution and finite volume on the black hole mass function, which in turn limits the mergers associated with low-mass black holes, M BH ≲ 10 6.5 M ⊙ . We find the upper limit on the total mass of the MBH binaries detectable by LISA to be ∼10 8.4 M ⊙ . We also find that adding time delays pertaining to dissipative processes like dynamical friction and stellar hardening during the final stages of the inspiral for which the simulation lacks sufficient resolution to model, considerably shifts the peak of the redshift distribution of detectable binaries from z ∼ 0.5 to z ∼ 0.1. Time delays reduce the number of detectable GW events, but on the other hand, their signal-to-noise is increased. From the observational point of view, we find a strong correlation between the star formation rate (SFR) and L bol at high redshifts for the detectable LISA binaries. This may prove to be a future application in the coincident observation of MBH binaries by GW and electromagnetic observations.