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) after central relaxation. Our results can be approximated by simple analytic scalings, with $M_{\rm IMBH} \propto v_{\rm cl}^{3/2}$ where $v_{\rm cl}^{2} = G\, M_{\rm cl}/r_{\rm h}$ 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 Mcl, below even the most conservative observational upper limits in all known cases. The IMBH mass could reach ≳ 104 M⊙ 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.

Formation of supermassive black holes in galactic nuclei – I. Delivering seed intermediate-mass black holes in massive stellar clusters

“…Our prescription for estimating the IMBH mass in our population synthesis results gives a mean IMBH mass of about 5×10 3 M and a median mass of about 1.5 × 10 3 M . These estimated masses are consistent with constraints derived from simulations of young massive clusters and semi-analytical estimates for BH growth in dense environments (Antonini et al 2019;Antonini & Gieles 2020;Shi et al 2021). Using the initial stellar cluster mass distribution from Schaerer & Charbonnel (2011) in our population synthesis approach, we found that most of the stellar clusters that aggregate to form the NSC have initial masses between ∼ 10 5.5 to ∼ 10 6.5 M .…”

Dynamics of intermediate-mass black holes wandering in the milky way galaxy using the illustris TNG50 simulation

“…Of course, other formation processes for IMBHs have been proposed, including: (i) runaway mergers in globular star clusters (e.g. Portegies Zwart & McMillan 2002 ;G ürkan, Freitag & Rasio 2004 ;Gonz ález et al 2021 ;Shi, Grudi ć & Hopkins 2021 ), (ii) hyper-Eddington accretion on to stellar-mass black holes (e.g. Ryu et al 2016 ), (iii) direct collapse of hypermassive quasi-stars (e.g.…”

Hyper-Eddington black hole growth in star-forming molecular clouds and galactic nuclei: can it happen?

“…Observations have demonstrated the existence of supermassive black holes (BHs) with masses Mbh ∼ 10 9 M in quasars at very high redshift (z 7) when the Universe was less than a billion years old (e.g., Fan et al 2001;Wang et al 2021), which implies that these BHs must accrete rapidly from their "seeds" (Inayoshi et al 2020). The physical origin of these seeds remains deeply uncertain, but popular models including direct collapse of super-massive stars with masses ∼ 10 4 − 10 6 M (e.g., Begelman et al 2006;Regan et al 2017;Corbett Moran et al 2018;Chon & Omukai 2020), runaway mergers in globular clusters (e.g., Portegies Zwart et al 2004;Boco et al 2020;Alister Seguel et al 2020;Kremer et al 2020;Rizzuto et al 2021;Shi et al 2021;Fragione et al 2021), remnants from Population III stars (e.g., Madau & Rees 2001;Ryu et al 2016), and relics of "standard" stellar evolution (e.g. Population II) stars generally produce seeds with masses 10 4 M .…”