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The mass budget for intermediate-mass black holes in dense star clusters

Shi, Yanlong and Grudić, Michael Y. and Hopkins, Philip F. (2021) The mass budget for intermediate-mass black holes in dense star clusters. Monthly Notices of the Royal Astronomical Society, 505 (2). pp. 2753-2763. ISSN 0035-8711. doi:10.1093/mnras/stab1470.

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Intermediate-mass black holes (IMBHs) could form via runaway merging of massive stars in a young massive star cluster (YMC). We combine a suite of numerical simulations of YMC formation with a semi-analytic model for dynamical friction and merging of massive stars and evolution of a central quasi-star, to predict how final quasi-star and relic IMBH masses scale with cluster properties (and compare with observations). The simulations argue that inner YMC density profiles at formation are steep (approaching isothermal), producing some efficient merging even in clusters with relatively low effective densities, unlike models that assume flat central profiles resembling those of globular clusters after central relaxation. Our results can be approximated by simple analytic scalings, with M_(IMBH) ∝ v^(3/2)_(cl) where v²_(cl) = GM_(cl)/r_h is the circular velocity in terms of initial cluster mass Mcl and half-mass radius rh. While this suggests IMBH formation is possible even in typical clusters, we show that predicted IMBH masses for these systems are small, ∼100−1000 M_⊙ or ∼0.0003 M_(cl) ⁠, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach ≳ 10⁴ M⊙ in the centres nuclear star clusters, ultra-compact dwarfs, or compact ellipticals, but in all these cases the prediction remains far below the present observed supermassive BH masses in these systems.

Item Type:Article
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URLURL TypeDescription Paper
Shi, Yanlong0000-0002-0087-3237
Grudić, Michael Y.0000-0002-1655-5604
Hopkins, Philip F.0000-0003-3729-1684
Additional Information:© 2021 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. Accepted 2021 May 18. Received 2021 April 21; in original form 2020 September 9. Published: 26 May 2021. We thank the anonymous reviewer whose comments and suggestions helped improve and clarify this manuscript. Support for YS and PFH was provided by NSF Collaborative Research Grants 1715847 and 1911233, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, JPL 1589742. Numerical calculations were run on the Caltech compute cluster ‘Wheeler,’ allocations FTA-Hopkins supported by the NSF and TACC, and NASA HEC SMD-16-7592. Data Availability Statements: The YMC simulation data underlying this article will be shared on reasonable request to the corresponding author, and the code for YMC analysis and Monte Carlo simulation is available at the GitHub repository yanlongastro/IMBH-runaway-mergers.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Subject Keywords:stars: formation – globular clusters: general – galaxies: formation – galaxies: star clusters: general
Issue or Number:2
Record Number:CaltechAUTHORS:20200922-124141959
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Official Citation:Yanlong Shi, Michael Y Grudić, Philip F Hopkins, The mass budget for intermediate-mass black holes in dense star clusters, Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 2, August 2021, Pages 2753–2763,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:105470
Deposited By: Tony Diaz
Deposited On:22 Sep 2020 20:29
Last Modified:02 Aug 2021 18:37

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