2025ApJ...986..163W

The detection of the gravitational-wave (GW) event GW190521 raises questions about the formation of black holes (BHs) within the pair-instability mass gap (PIBHs). We propose that Population III star clusters significantly contribute to events similar to GW190521. We perform N-body simulations and find that PIBHs can form from stellar collisions or binary black hole (BBH) mergers, with the latter accounting for 90% of the contributions. Due to GW recoil during BBH mergers, approximately 10%-50% of PIBHs formed via BBH mergers that escaped from clusters, depending on BH spins and cluster escape velocities. The remaining PIBHs can participate in secondary and multiple BBH formation events, contributing to GW events. Assuming Population III stars form in massive clusters (initially 100,000 M_⊙) with a top-heavy initial mass function, the average merger rates for GW events involving PIBHs with 0% and 100% primordial binaries are 0.005 and 0.017 yr^–1 Gpc^–3, respectively, with maximum values of 0.030 and 0.106 yr^–1 Gpc^–3. If Population III stars form in low-mass clusters (initial mass of 1000 M_⊙ and 10,000 M_⊙), the merger rate is comparable with a 100% primordial binary fraction but significantly lower without primordial binaries. We also calculate the characteristic strains of the GW events in our simulations and find that about 43.4% (LISA), 97.8% (Taiji), and 66.4% (Tianqin) of these events could potentially be detected by space-borne detectors, including LISA, Taiji, and TianQin. Next-generation GW detectors such as DECIGO, the Einstein Telescope, and Cosmic Explorer can nearly cover all these signals.