Gravitational wave of intermediate-mass black holes in Population III star clusters

TL;DR

This study uses detailed N-body simulations to show that Population III star clusters can produce intermediate-mass black holes and gravitational wave events detectable across a wide range of redshifts, including high-redshift mergers.

Contribution

The paper presents the first star-by-star N-body simulation analysis of Pop III star clusters, demonstrating their potential to produce IMBHs and gravitational wave events observable by current and future detectors.

Findings

Pop III star clusters can produce IMBH-BH mergers from redshift 0 to 20.

Estimated GW event rate is 0.1-0.8 per year per Gpc^3.

Most mergers are detectable by DECIGO and advanced LIGO/VIRGO/Kagra.

Abstract

Previous theoretical studies suggest that the Population III (Pop3) stars tend to form in extremely metal poor gas clouds with approximately $10^5 M_\odot$ embedded in mini dark matter halos. Very massive stars can form via multiple collisions in Pop3 star clusters and eventually evolve to intermediate-mass black holes (IMBHs). In this work, we conduct star-by-star $N$-body simulations for modelling the long-term evolution of Pop3 star clusters. We find that if the mini dark matter halos can survive today, these star clusters can avoid tidal disruption by the galactic environment and can efficiently produce IMBH-BH mergers among a wide range of redshift from 0 to 20. The average gravitational wave event rate is estimated to be $0.1-0.8~\mathrm{yr}^{-1} \mathrm{Gpc}^{-3}$, and approximately $40-80$ percent of the mergers occur at high redshift ($z>6$). The characteristic strain shows that a part of low-redshift mergers can be detected by LISA, TianQin, and Taiji, whereas most mergers can be covered by DECIGO and advanced LIGO/VIRGO/Kagra. Mergers with pair-instability BHs have a rate of approximately $0.01-0.15$~yr$^{-1}$~Gpc$^{-3}$, which can explain the GW190521-like events.