Binary Black Hole Mergers Reveal Secrets of Early Universe’s Population III Stars

The formation of binary black holes (BBHs) from Population III star clusters is a crucial aspect of understanding the early universe. These BBHs are expected to be one of the main targets for next-generation ground-based gravitational wave detectors. The predicted initial mass function and lack of metals make them ideal progenitors of black holes above the upper edge of the pair-instability mass gap.

Can Binary Black Hole Mergers Reveal Secrets of the Early Universe?

The formation of binary black holes (BBHs) from Population III star clusters is a crucial aspect of understanding the early universe. These BBHs are expected to be one of the main targets for next-generation ground-based gravitational wave detectors. The predicted initial mass function and lack of metals make them ideal progenitors of black holes above the upper edge of the pair-instability mass gap.

In this article, we will delve into the effects of cluster dynamics on the mass function of BBHs born from Pop III stars. We will consider the main uncertainties on Pop III star mass function, orbital properties of binary systems, star clusters, and disruption time. Our dynamical models suggest that at least 5 and up to 100 BBH mergers in Pop III star clusters have primary masses above the upper edge of the pair-instability mass gap.

What Drives the Formation of Binary Black Holes?

The formation of BBHs is a complex process that involves the evolution of Population III stars. These stars are thought to have formed from zero-metallicity gas, and their initial mass function is still uncertain. The lack of metals in these stars makes them ideal progenitors of black holes above the upper edge of the pair-instability mass gap.

In our models, we consider the main uncertainties on Pop III star mass function, orbital properties of binary systems, star clusters, and disruption time. We find that at least 5 and up to 100 BBH mergers in Pop III star clusters have primary masses above the upper edge of the pair-instability mass gap.

The Role of Cluster Dynamics

Cluster dynamics plays a crucial role in shaping the mass function of BBHs born from Pop III stars. Our models suggest that the main uncertainties on Pop III star mass function, orbital properties of binary systems, and disruption time all contribute to the formation of BBH mergers.

We find that at least 5 and up to 100 BBH mergers in Pop III star clusters have primary masses above the upper edge of the pair-instability mass gap. In contrast, only 3 isolated BBH mergers have primary masses above the gap unless their progenitors evolved as chemically homogeneous stars.

The Zone of Avoidance

The lack of systems with primary and/or secondary masses inside the gap defines a zone of avoidance with sharp boundaries in the primary mass-mass ratio plane. This zone is characterized by a dearth of BBH mergers with primary masses below the upper edge of the pair-instability mass gap.

Our models suggest that this zone is a result of the interplay between cluster dynamics and the initial mass function of Pop III stars. The formation of BBHs above the upper edge of the pair-instability mass gap is favored by the lack of metals in these stars, which makes them ideal progenitors of black holes with masses higher than 134,241 M.

Estimating the Merger Rate Density

We estimate the merger rate density of BBHs and find that in the most optimistic case, we can reach a maximum of R = 200 Gpc-3 yr-1 for BBHs formed via dynamical capture. For comparison, the merger rate density of isolated Pop III BBHs is R = 10 Gpc-3 yr-1 for the same model of Pop III star formation history.

Conclusion

In conclusion, our models suggest that cluster dynamics plays a crucial role in shaping the mass function of BBHs born from Pop III stars. The lack of systems with primary and/or secondary masses inside the gap defines a zone of avoidance with sharp boundaries in the primary mass-mass ratio plane.

Our estimates suggest that the merger rate density of BBHs formed via dynamical capture can reach a maximum of R = 200 Gpc-3 yr-1, which is significantly higher than the merger rate density of isolated Pop III BBHs. These results highlight the importance of considering cluster dynamics in understanding the formation and evolution of BBHs.