Jaime Salcido, Richard G. Bower, Tom Theuns, Stuart McAlpine, Matthieu Schaller, Robert A. Crain, Joop Schaye, John Regan, Durham University, (2) Liverpool John Moores, (3) Leiden Observatory)

We estimate the expected event rate of gravitational wave signals from mergers of supermassive black holes that could be resolved by a space-based interferometer, such as the Evolved Laser Interferometer Space Antenna (eLISA), utilising cosmological hydrodynamical simulations from the EAGLE suite. These simulations assume a $\Lambda$CDM cosmogony with state-of-the-art subgrid models for radiative cooling, star formation, stellar mass loss, and feedback from stars and accreting black holes. They have been shown to reproduce the observed galaxy population with unprecedented fidelity. We combine the merger rates of supermassive black holes in EAGLE with a model to calculate the gravitational waves signals from the intrinsic parameters of the black holes. The EAGLE models predict $\sim2$ detections per year by a gravitational wave detector such as eLISA. We find that these signals are largely dominated by mergers between $10^5 \textrm{M}_{\odot} h^{-1}$ seed mass black holes merging at redshifts between $z\sim2.5$ and $z\sim0.5$. In order to investigate the dependence on the assumed black hole seed mass, we introduce an additional model with black hole seed mass an order of magnitude smaller than in our reference model. We find that the merger rate is similar in both models, but that the scenarios could be distinguished through their detected gravitational waveforms. Hence, the characteristic gravitational wave signals detected by eLISA will provide profound insight into the origin of supermassive black holes and the initial mass distribution of black hole seeds.