Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds

Chattopadhyay, Debatri, Stegmann, Jakob, Antonini, Fabio ORCID: https://orcid.org/0000-0003-3138-6199, Barber, Jordan and Romero-Shaw, Isobel M. 2023. Double black hole mergers in nuclear star clusters: eccentricities, spins, masses, and the growth of massive seeds. Monthly Notices of the Royal Astronomical Society 526 (4) , 4908–4928. 10.1093/mnras/stad3048

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Abstract

We investigate the formation of intermediate mass black holes (IMBHs) through hierarchical mergers of stellar origin black holes (BHs), as well as BH mergers formed dynamically in nuclear star clusters. Using a semi-analytical approach that incorporates probabilistic mass-function-dependent double BH (DBH) pairing, binary-single encounters, and a mass-ratio-dependent prescription for energy dissipation in hardening binaries, we find that IMBHs with masses of – can be formed solely through hierarchical mergers in time-scales of a few 100 Myrs to a few Gyrs. Clusters with escape velocities ≳400 km s−1 inevitably form high-mass IMBHs. The spin distribution of IMBHs with masses ≳ 103 M⊙ is strongly clustered at χ ∼ 0.15; while for lower masses, it peaks at χ ∼ 0.7. Eccentric mergers are more frequent for equal-mass binaries containing first- and/or second-generation BHs. Metal-rich, young, dense clusters can produce up to 20 per cent of their DBH mergers with eccentricity ≥0.1 at ⁠, and ∼2–9 per cent of all in-cluster mergers can form at >10 Hz. Nuclear star clusters are therefore promising environments for the formation of highly eccentric DBH mergers, detectable with current gravitational-wave detectors. Clusters of extreme mass (∼108 M⊙) and density (∼108 M⊙ pc−3) can have about half of all of their DBH mergers with primary masses ≥100 M⊙. The fraction of in-cluster mergers increases rapidly with increasing cluster escape velocity, being nearly unity for vesc ≳ 200 km s−1. Cosmological merger rate of DBHs from nuclear clusters varies ⪅0.01–1 Gpc−3 yr−1, where the large error bars come from uncertainties in the cluster initial conditions, number density distribution, and redshift evolution of nucleated galaxies.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Physics and Astronomy
Publisher:	Oxford University Press
ISSN:	0035-8711
Funders:	STFC
Date of First Compliant Deposit:	25 January 2024
Date of Acceptance:	3 October 2023
Last Modified:	10 Apr 2024 15:41
URI:	https://orca.cardiff.ac.uk/id/eprint/165807

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