Mass and Rate of Hierarchical Black Hole Mergers in Young, Globular and Nuclear Star Clusters

Michela Mapelli,Yann Bouffanais,Alessandro Ballone,Filippo Santoliquido,Manuel Arca Sedda,Maria Celeste Artale

doi:10.3390/sym13091678

Michela Mapelli, Yann Bouffanais + Show 4 more

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https://doi.org/10.3390/sym13091678

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Abstract

Hierarchical mergers are one of the distinctive signatures of binary black hole (BBH) formation through dynamical evolution. Here, we present a fast semi-analytic approach to simulate hierarchical mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs). Hierarchical mergers are more common in NSCs than they are in both GCs and YSCs because of the different escape velocity. The mass distribution of hierarchical BBHs strongly depends on the properties of first-generation BBHs, such as their progenitor’s metallicity. In our fiducial model, we form black holes (BHs) with masses up to ∼103 M⊙ in NSCs and up to ∼102 M⊙ in both GCs and YSCs. When escape velocities in excess of 100 km s−1 are considered, BHs with mass >103 M⊙ are allowed to form in NSCs. Hierarchical mergers lead to the formation of BHs in the pair instability mass gap and intermediate-mass BHs, but only in metal-poor environments. The local BBH merger rate in our models ranges from ∼10 to ∼60 Gpc−3 yr−1; hierarchical BBHs in NSCs account for ∼10−2–0.2 Gpc−3 yr−1, with a strong upper limit of ∼10 Gpc−3 yr−1. When comparing our models with the second gravitational-wave transient catalog, we find that multiple formation channels are favored to reproduce the observed BBH population.

Highlights

The past six years have witnessed the first three observing runs of the AdvancedLIGO and Virgo gravitational wave (GW) interferometers [1,2], leading to the detection of about 50 binary compact object mergers [3,4,5,6,7,8,9,10,11]
The maximum primary and secondary mass strongly depend on the environment: We have 2g binary black hole (BBH) even in young star clusters (YSCs) and globular clusters (GCs), but nuclear star clusters (NSCs) are more effective in producing hierarchical mergers because of the larger value of vesc
Hierarchical mergers in dynamical environments can lead to the formation of black holes (BHs) with mass higher than the limits imposed by pair instability, core-collapse supernovae and stellar evolution theory

Summary

Introduction

LIGO and Virgo gravitational wave (GW) interferometers [1,2], leading to the detection of about 50 binary compact object mergers [3,4,5,6,7,8,9,10,11] This growing sample of GW observations represents a “Rosetta stone” to investigate the formation of binary compact objects. One of the distinctive signatures of the dynamical scenario is the formation of hierarchical mergers, i.e., repeated mergers of stellar-origin black holes (BHs) that build up more massive ones [84,85,86,87,88]. The main obstacle to the formation of second-generation (2g) BHs via hierarchical mergers is the high relativistic kick that the merger remnant receives at birth because of radiation of linear momentum through beamed GW emission (e.g., [90,91,92,93])

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