Milky Way Globular Clusters: Nurseries for Dynamically-Formed Binary Black Holes

TL;DR

This paper develops a comprehensive theoretical framework combining galaxy formation and cluster evolution models to study the origins and merger sites of black hole binaries, with implications for future gravitational wave detectors.

Contribution

It introduces a novel self-consistent model linking globular cluster evolution with black hole mergers, explaining observed distributions and high-redshift merger rates.

Findings

Approximately 30% of Milky Way halo globular clusters originate from satellite galaxies.

Hierarchical black hole mergers significantly contribute to the black hole mass spectrum.

Merger and birth rates of binary black holes increase with redshift up to z=5.

Abstract

We present a novel self-consistent theoretical framework to characterize the formation, evolution, and merger sites of dynamically-formed black hole binaries, with a focus on explaining the most massive events observed by the LIGO-Virgo-KAGRA Collaboration. Our approach couples the galaxy formation model GAMESH with cluster population synthesis codes to trace the cosmic evolution of globular clusters simultaneously with mergers of massive black holes. Our reference model, which includes prescriptions for both cluster formation and disruption depending on properties of specific galaxies, accurately reproduces the observed age-mass distribution of the Milky Way globular clusters. We find that approximately 30% of the globular clusters observed in our galaxy's halo may have originated from satellite galaxies of the Milky Way. We confirm that hierarchical black hole mergers provide a significant contribution to the formation of black holes in and above the pair-instability mass gap. However, quantifying their contribution is challenging, as different population synthesis codes yield divergent results in terms of black hole mass function and merger rates. Furthermore, we characterize the host galaxies where massive black holes form in terms of their dark matter, stellar mass, and metallicity. Ultimately, we demonstrate that the merger and birth rate densities of binary black holes increase with redshift till z = 5. This cosmic evolution is a crucial signature with significant implications for future detectors like the LISA, the Einstein Telescope and Cosmic Explorer, which will be capable to probe the high-redshift Universe.