MOCCA-SURVEY Database I: Coalescing Binary Black Holes Originating From Globular Clusters

Abstract: In this first of a series of papers, we utilize results for around two thousand star cluster models simulated using the MOCCA code for star cluster evolution (Survey Database I) to determine the astrophysical properties and local merger rate densities for coalescing binary black holes (BBHs) originating from globular clusters (GCs). We extracted information for all coalescing BBHs that escape the cluster models and subsequently merge within a Hubble time along with BBHs that are retained in our GC models and merge inside the cluster via gravitational wave (GW) emission. By obtaining results from a substantial number of realistic star cluster models that cover different initial parameters, we have an extremely large statistical sample of BBHs with stellar mass and massive stellar BH ($\lesssim 100M_{\odot}$) components that merge within a Hubble time. Using this data, we estimate local merger rate densities for these BBHs originating from GCs to be at least 5.4 ${\rm Gpc}^{-3}\,{\rm yr}^{-1}$

• The paper estimates a local merger rate density of at least 5.4 Gpc⁻³ yr⁻¹ for BBHs using over 2000 MOCCA globular cluster simulations.
• It finds that early intense dynamic interactions and lower metallicity in clusters significantly increase BBH mergers within a Hubble time.
• The study highlights that multiple stellar encounters in globular clusters are key to forming and hardening binary black holes.

Coalescing Binary Black Holes from Globular Clusters: An Analysis Using the MOCCA-Survey Database I

The study by Askar et al. presents the first comprehensive analysis of binary black hole (BBH) mergers originating from globular clusters (GCs) using results from approximately two thousand star cluster models simulated using the MOCCA code. The primary objective is to determine the astrophysical characteristics and local merger rate densities for these BBHs by leveraging a statistically significant dataset that encapsulates a wide array of initial GC parameters.

Research Methodology

The authors utilize the MOCCA (MOnte Carlo Cluster simulAtor) code, renowned for its efficiency and detailed outputs. This code allows for sophisticated simulation of star clusters, including key dynamical processes and stellar evolution parameters. The simulated models vary in terms of initial conditions such as stellar populations, metallicity, binary fraction, and more, providing a broad representation of potential real-world initial conditions in GCs.

Key Findings

1. Merger Rate Density: The study estimates a local merger rate density of at least 5.4 ${\rm Gpc}^{-3}\,{\rm yr}^{-1}$ for BBHs originating from GCs. This value aligns with the lower bound of LIGO's observed rates and suggests a notable contribution from dynamical processes within GCs to the observed BBH merger events.
2. Mass and Time Distributions: It was observed that the peak merger events occur early in the GC lifetime due to intense dynamical interactions, with subsequent mergers following a decreasing trend. Models with lower metallicity produce more BBHs that merge within a Hubble time compared to those with higher metallicity, consistent with the theoretical expectations of massive stellar evolution.
3. Dynamic Interactions: The simulations confirm that dynamic interactions within GCs are pivotal for forming BBHs, as these environments enable multiple stellar encounters and BBH hardening.

Implications and Future Directions

The study underscores the importance of GCs in contributing to the population of merging BBHs detected by gravitational wave observatories. The results provide a theoretical basis which, combined with observations, can refine the understanding of BBH formation channels. This is particularly crucial as LIGO and other detectors expand their observation capabilities, offering more data to compare against predictions from models like those produced by MOCCA.

In future work, extending simulations to include even more massive clusters and accounting for more detailed stellar evolution scenarios and metallicity distributions could refine merger rate density estimates. Improvements in these areas have the potential to more precisely match observational data, shedding light on the contributions of different astrophysical environments to the observed gravitational wave events.

In conclusion, Askar et al.'s analysis contributes a robust quantitative framework for understanding the role of globular clusters in producing BBH mergers, with significant implications for both theoretical astrophysics and observational strategies in the era of gravitational wave astronomy.