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

Abstract

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 which assume flat central profiles resembling those of globular clusters (GCs) {\em after} central relaxation. Our results can be approximated by simple analytic scalings, with $M_{\mathrm{IMBH}}\propto v_{\mathrm{cl}}^{3/2}$ where $v_{\mathrm{cl}}^2=G{M}_{\mathrm{cl}}/r_{\mathrm{h}}$ is the circular velocity in terms of initial cluster mass $M_{\mathrm{cl}}$ and half-mass radius $r_{\mathrm{h}}$. While this suggests IMBH formation is {\em possible} even in typical clusters, we show that predicted IMBH masses for these systems are small, $\sim 100-1000M_{\odot }$ or $\sim 0.0003M_{\mathrm{cl}}$, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach $\gtrsim 10^4{M}_{\odot }$ in the centers 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.

10 pages, 6 figures. Comments welcomed!

References in corpus (16)

• Formation of the first nuclear clusters and massive black holes at high redshift
• Observational evidence for intermediate-mass black holes
• N-body modeling of globular clusters: Masses, mass-to-light ratios and intermediate-mass black holes
• The Luminosity Functions and Timescales of MYSOs and Compact HII regions
• Gravitational Wave Recoil and the Retention of Intermediate Mass Black Holes
• Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
• Super Star Cluster Velocity Dispersions and Virial Masses in the M82 Nuclear Starburst
• Dynamical masses of ultra-compact dwarf galaxies in Fornax
• Intermediate Mass Black Hole Formation in compact Young Massive Star Clusters
• Bright globular clusters in NGC 5128: the missing link between young massive clusters and evolved massive objects
• Evidence for an intermediate-mass black hole in the globular cluster NGC 6624
• The global mass functions of 35 Galactic globular clusters: I. Observational data and correlations with cluster parameters
• Sizes and Shapes of Young Star Cluster Light Profiles in M83
• Evolution of giant molecular clouds across cosmic time
• Mass models of NGC 6624 without an intermediate-mass black hole
• Dynamical friction in multi-component evolving globular clusters

Cited by in corpus (14)

• FIRE-3: Updated Stellar Evolution Models, Yields, & Microphysics and Fitting Functions for Applications in Galaxy Simulations
• Seeds Don't Sink: Even Massive Black Hole "Seeds" Cannot Migrate to Galaxy Centers Efficiently
• A search for active galactic nuclei in low-mass compact galaxies
• A New Discrete Dynamical Friction Estimator Based on $N$-body Simulations
• Hyper-Eddington Black Hole Growth in Star-Forming Molecular Clouds and Galactic Nuclei: Can It Happen?
• Formation of supermassive black holes in galactic nuclei II: retention and growth of seed intermediate-mass black holes
• Dynamics of Intermediate-Mass Black Holes Wandering in the Milky Way Galaxy Using the Illustris TNG50 Simulation
• Detectability of Wandering Intermediate-Mass Black Holes in the Milky Way Galaxy from Radio to X-rays
• From Seeds to Supermassive Black Holes: Capture, Growth, Migration, and Pairing in Dense Proto-Bulge Environments
• Feedback-regulated Seed Black Hole Growth in Star-Forming Molecular Clouds and Galactic Nuclei
• Three-dimensional radiation hydrodynamics simulations of wandering intermediate-mass black holes considering the anisotropic radiation and dust sublimation
• Cosmic wallflowers: the circumgalactic origins of isolated ultra-compact star clusters at $z>7$
• An Accurate Comprehensive Approach to Substructure: IV. Dynamical Friction
• The In Situ Growth of Stellar-mass "Light" Seed Black Holes in Nuclear Star Clusters