Music from the heavens - Gravitational waves from supermassive black hole mergers in the EAGLE simulations

TL;DR

This paper predicts that space-based gravitational wave detectors like eLISA could observe about two supermassive black hole mergers annually, providing insights into black hole origins and seed masses through waveform analysis.

Contribution

It combines cosmological simulations with waveform models to estimate detection rates and explores how initial black hole seed mass affects gravitational wave signals.

Findings

Approximately 2 detections per year predicted by eLISA.

Mergers mainly involve seed black holes at redshifts 1-2.

Waveform differences can reveal initial black hole seed masses.

Abstract

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 the reference cosmological hydrodynamical simulation 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 the latest phenomenological waveform models to calculate the gravitational waves signals from the intrinsic parameters of the merging 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 seed mass black holes merging at redshifts between $z\sim2$ and $z\sim1$. In order to investigate the dependence on the assumed black hole seed mass, we introduce an additional model with a black hole seed mass an order of magnitude smaller than in our reference model. We also consider a variation of the reference model where a prescription for the expected delays in the black hole merger timescale has been included after their host galaxies merge. We find that the merger rate is similar in all models, but that the initial black hole seed mass 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.