Rapid intermediate-mass black hole formation via runaway mergers of black holes
Pith reviewed 2026-06-27 16:26 UTC · model grok-4.3
The pith
Dense black hole clusters form 1000-solar-mass objects via runaway gravitational-wave mergers in under 10 million years.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using N-body simulations, the authors demonstrate that in dense stellar-mass black hole clusters with densities greater than or equal to 5 times 10 to the 9 solar masses per cubic parsec, a chain of exchanged soft binary black holes undergoes accumulated hardening leading to runaway gravitational-wave mergers that produce an intermediate-mass black hole of approximately 1000 solar masses within 10 million years.
What carries the argument
The runaway chain of gravitational-wave binary black hole mergers driven by exchange of soft binaries with accumulated hardening, which proves more efficient than three-body scattering.
Load-bearing premise
Black hole subsystems must reach and sustain densities of at least 5 times 10 to the 9 solar masses per cubic parsec long enough for the exchange-hardening-merger chain to operate before cluster expansion quenches it.
What would settle it
An N-body simulation initialized at the required densities but including realistic expansion from the start that produces no intermediate-mass black hole would settle the claim.
Figures
read the original abstract
Observations indicate that supermassive black holes (SMBHs) in high-redshift galaxies formed on timescales far shorter than classical growth models allow. One hypothesis suggests intermediate-mass black hole (IMBH) seeds as an efficient growth channel. Using N-body simulations, we demonstrate that in dense stellar-mass black hole (BH) clusters ($\ge 5\times10^9 M_{\odot}/{\rm pc}^3$), runaway gravitational-wave binary BH (BBH) mergers can produce a $\sim 10^3 M_\odot$ IMBH within 10 Myr from the formation of the BH subsystem. This scenario is simple and avoids large uncertainties regarding stellar mergers and evolution in the IMBH formation via very massive stars channel. We find that the runaway GW-merger mechanism relies on hard BBH formation through a chain of exchanged soft BBHs with accumulated hardening, which is far more efficient than three-body scattering. We analyze how IMBH formation depends on cluster density, total mass, initial mass function, and stellar halo potential. We find that due to cluster expansion, the systems forming IMBHs have densities consistent with present-day nuclear star clusters, such as those in the Milky Way and M33. Furthermore, we show that IMBH spin remains low due to repeated mergers, and we estimate the rate of GW190521 and GW231123-like events within the first 100 Myr to be $2.27-247.52$ and $3.23-63.63 $ per Gyr per cluster.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses N-body simulations to demonstrate that in dense stellar-mass black hole clusters with initial densities ≥5×10^9 M⊙/pc³, a chain of exchanged and hardened BBH mergers via gravitational waves can produce a ∼10³ M⊙ IMBH within 10 Myr from BH-subsystem formation. It examines dependence on density, total mass, IMF, and halo potential; reports consistency with nuclear star cluster densities; notes low IMBH spin from repeated mergers; and estimates rates for GW190521- and GW231123-like events.
Significance. If the extreme initial densities prove attainable, the work supplies a relatively simple IMBH-seed channel that avoids stellar-merger uncertainties and yields falsifiable predictions for early GW-event rates and IMBH spins. The reported consistency between simulated densities and observed nuclear clusters is a concrete strength.
major comments (2)
- [Abstract and initial conditions] Abstract and initial-conditions section: the central claim requires that BH subsystems can reach and sustain densities ≥5×10^9 M⊙/pc³ long enough for the exchange-hardening-merger sequence to complete within 10 Myr, yet the simulations initialize at this density threshold rather than deriving it from the prior evolution of a realistic parent cluster that includes stellar evolution, mass segregation, and gas expulsion.
- [Numerical methods] Numerical-methods description (presumably § on simulation setup): the manuscript supplies no information on the N-body integrator, softening prescription, post-Newtonian terms, or validation against known three-body or GW-merger benchmarks; without these the reliability of the reported runaway-merger chain cannot be assessed.
minor comments (1)
- [Rate estimates] The quoted rate ranges (2.27–247.52 and 3.23–63.63 per Gyr per cluster) are broad; a brief statement of the parameter variations that produce the bounds would aid interpretation.
Simulated Author's Rebuttal
We thank the referee for their constructive and detailed report. We address each major comment below and outline the revisions we will make.
read point-by-point responses
-
Referee: Abstract and initial-conditions section: the central claim requires that BH subsystems can reach and sustain densities ≥5×10^9 M⊙/pc³ long enough for the exchange-hardening-merger sequence to complete within 10 Myr, yet the simulations initialize at this density threshold rather than deriving it from the prior evolution of a realistic parent cluster that includes stellar evolution, mass segregation, and gas expulsion.
Authors: The referee is correct that the simulations are initialized with the BH subsystem already at the stated density rather than evolved from a full parent cluster that includes gas expulsion, stellar evolution, and mass segregation. The scope of the present work is the subsequent dynamical evolution of the BH subsystem once formed, as the preceding phases involve substantial additional uncertainties not addressed here. We will add an expanded discussion in the introduction and a dedicated paragraph in the conclusions that cites existing literature on mass segregation and core collapse leading to dense BH subsystems, while explicitly stating the assumption that such densities can be reached. No new simulations of the parent cluster will be performed. revision: partial
-
Referee: Numerical-methods description (presumably § on simulation setup): the manuscript supplies no information on the N-body integrator, softening prescription, post-Newtonian terms, or validation against known three-body or GW-merger benchmarks; without these the reliability of the reported runaway-merger chain cannot be assessed.
Authors: We agree that the numerical-methods section lacks the required technical details. In the revised manuscript we will add a dedicated methods subsection specifying the N-body integrator, the softening prescription, the post-Newtonian terms included, and the validation tests performed against three-body scattering and known GW-merger benchmarks. revision: yes
Circularity Check
No significant circularity; results are direct N-body outcomes from stated initial conditions.
full rationale
The paper reports N-body simulation results showing that, given initial BH subsystem densities ≥5×10^9 M⊙/pc³, runaway GW mergers can form ~10³ M⊙ IMBHs in ≤10 Myr. This is a forward integration of explicitly supplied initial conditions (density, mass, IMF, halo potential) rather than any re-expression of fitted outputs, self-defined quantities, or load-bearing self-citations. The density threshold is presented as a prerequisite, not derived within the paper, and no equations or claims reduce the reported formation times or rates to tautological inputs by construction. The derivation chain is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
free parameters (2)
- minimum initial density threshold =
5e9 M⊙ pc^{-3}
- initial mass function parameters
axioms (2)
- standard math Newtonian gravity plus post-Newtonian corrections for gravitational-wave emission accurately capture the binary hardening and merger dynamics
- domain assumption Black-hole subsystems can form with the required initial densities before significant expansion occurs
Reference graph
Works this paper leans on
-
[1]
2026, JCAP, 2026, 081, doi: 10.1088/1475-7516/2026/03/081
Abac, A., Abramo, R., Albanesi, S., et al. 2026, JCAP, 2026, 081, doi: 10.1088/1475-7516/2026/03/081
-
[2]
G., Abouelfettouh, I., Acernese, F., et al
Abac, A. G., Abouelfettouh, I., Acernese, F., et al. 2025, ApJL, 993, L25, doi: 10.3847/2041-8213/ae0c9c
-
[3]
GW190521: A Binary Black Hole Merger with a Total Mass of 150 M⊙.Phys
Abbott, R., Abbott, T. D., Abraham, S., et al. 2020a, PhRvL, 125, 101102, doi: 10.1103/PhysRevLett.125.101102
-
[4]
Abbott, R., Abbott, T. D., Abraham, S., et al. 2020b, ApJL, 900, L13, doi: 10.3847/2041-8213/aba493
-
[5]
Abbott, R., Abbott, T. D., Acernese, F., et al. 2022, A&A, 659, A84, doi: 10.1051/0004-6361/202141452 Abdurro’uf, Coe, D., Resseguier, T., et al. 2025, arXiv e-prints, arXiv:2512.08054, doi: 10.48550/arXiv.2512.08054
-
[6]
Adamo, A., Bradley, L. D., Vanzella, E., et al. 2024, Nature, 632, 513, doi: 10.1038/s41586-024-07703-7
-
[7]
Akins, H. B., Casey, C. M., Berg, D. A., et al. 2025, ApJL, 980, L29, doi: 10.3847/2041-8213/adab76
-
[8]
2014, Science, 345, 1330, doi: 10.1126/science.1251053
Alexander, T., & Natarajan, P. 2014, Science, 345, 1330, doi: 10.1126/science.1251053
-
[9]
Alvarez, M. A., Wise, J. H., & Abel, T. 2009, ApJL, 701, L133, doi: 10.1088/0004-637X/701/2/L133
-
[10]
2019, Monthly Notices of the Royal Astronomical Society, 486, 50085021, doi: 10.1093/mnras/stz1149
Antonini, F., Gieles, M., & Gualandris, A. 2019, Monthly Notices of the Royal Astronomical Society, 486, 50085021, doi: 10.1093/mnras/stz1149
-
[11]
Antonini, F., & Rasio, F. A. 2016, ApJ, 831, 187, doi: 10.3847/0004-637X/831/2/187
-
[12]
Apostolatos, T. A., Cutler, C., Sussman, G. J., & Thorne, K. S. 1994, Physical Review D, 49, 6274, doi: 10.1103/PhysRevD.49.6274
-
[13]
Askar, A., Vergara, M. C., & Ali, S. 2025, arXiv e-prints, arXiv:2510.03766, doi: 10.48550/arXiv.2510.03766 19 Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A, 558, A33, doi: 10.1051/0004-6361/201322068 Astropy Collaboration, Price-Whelan, A. M., Sipőcz, B. M., et al. 2018, AJ, 156, 123, doi: 10.3847/1538-3881/aabc4f Astropy Co...
-
[14]
Atallah, D., Weatherford, N. C., Trani, A. A., & Rasio, F. 2024, On Binary Formation from Three Initially Unbound Bodies, https://arxiv.org/abs/2402.12429 Bañados, E., Venemans, B. P., Decarli, R., et al. 2016, ApJS, 227, 11, doi: 10.3847/0067-0049/227/1/11 Bañados, E., Venemans, B. P., Mazzucchelli, C., et al. 2018, Nature, 553, 473, doi: 10.1038/nature25180
-
[15]
Banerjee, S., Belczynski, K., Fryer, C. L., et al. 2020, A&A, 639, A41, doi: 10.1051/0004-6361/201935332
-
[16]
1986, Nature, 324, 446, doi: 10.1038/324446a0
Barnes, J., & Hut, P. 1986, Nature, 324, 446, doi: 10.1038/324446a0
-
[17]
Begelman, M. C. 1979, MNRAS, 187, 237, doi: 10.1093/mnras/187.2.237
-
[18]
Begelman, M. C. 2010, MNRAS, 402, 673, doi: 10.1111/j.1365-2966.2009.15916.x
-
[19]
MNRAS , archivePrefix = "arXiv", eprint =
Begelman, M. C., Rossi, E. M., & Armitage, P. J. 2008, MNRAS, 387, 1649, doi: 10.1111/j.1365-2966.2008.13344.x
-
[20]
Begelman, M. C., Volonteri, M., & Rees, M. J. 2006, MNRAS, 370, 289, doi: 10.1111/j.1365-2966.2006.10467.x
-
[21]
1987, Galactic dynamics (Princeton University Press)
Binney, J., & Tremaine, S. 1987, Galactic dynamics (Princeton University Press)
1987
-
[22]
Blandford, R. D., & Begelman, M. C. 2004, MNRAS, 349, 68, doi: 10.1111/j.1365-2966.2004.07425.x Bogdán, Á., Goulding, A. D., Natarajan, P., et al. 2024, Nature Astronomy, 8, 126, doi: 10.1038/s41550-023-02111-9
-
[23]
galpy: A Python Library for Galactic Dynamics
Bovy, J. 2015, ApJS, 216, 29, doi: 10.1088/0067-0049/216/2/29
work page internal anchor Pith review doi:10.1088/0067-0049/216/2/29 2015
-
[24]
Breen, P. G., & Heggie, D. C. 2013, MNRAS, 432, 2779, doi: 10.1093/mnras/stt628
-
[25]
2013, Reports on Progress in Physics, 76, 112901, doi: 10.1088/0034-4885/76/11/112901
Bromm, V. 2013, Reports on Progress in Physics, 76, 112901, doi: 10.1088/0034-4885/76/11/112901
-
[26]
Bromm, V., & Larson, R. B. 2004, ARA&A, 42, 79, doi: 10.1146/annurev.astro.42.053102.134034
-
[27]
2003, The Astrophysical Journal, 596, 34, doi: 10.1086/377529
Bromm, V., & Loeb, A. 2003, ApJ, 596, 34, doi: 10.1086/377529
-
[28]
2007, The Astrophysical Journal, 659, L5L8, doi: 10.1086/516712
Campanelli, M., Lousto, C., Zlochower, Y., & Merritt, D. 2007, The Astrophysical Journal, 659, L5L8, doi: 10.1086/516712
-
[29]
Colpi, M., Danzmann, K., Hewitson, M., et al. 2024, arXiv e-prints, arXiv:2402.07571, doi: 10.48550/arXiv.2402.07571
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2402.07571 2024
-
[30]
Decarli, R., Walter, F., Venemans, B. P., et al. 2018, ApJ, 854, 97, doi: 10.3847/1538-4357/aaa5aa
-
[31]
Li, Z. 2023, MNRAS, 523, 3201, doi: 10.1093/mnras/stad1557
-
[32]
2025, arXiv e-prints, arXiv:2511.07578, doi: 10.48550/arXiv.2511.07578
Dekel, A., Dutta Chowdhury, D., Lapiner, S., et al. 2025, arXiv e-prints, arXiv:2511.07578, doi: 10.48550/arXiv.2511.07578
-
[33]
2019, ApJL, 870, L11, doi: 10.3847/2041-8213/aaeffe
Fan, X., Wang, F., Yang, J., et al. 2019, ApJL, 870, L11, doi: 10.3847/2041-8213/aaeffe
-
[34]
Fragione, G., Loeb, A., Kocsis, B., & Rasio, F. A. 2022, ApJ, 933, 170, doi: 10.3847/1538-4357/ac75d0
-
[35]
Freitag, M., Gürkan, M. A., & Rasio, F. A. 2006, MNRAS, 368, 141, doi: 10.1111/j.1365-2966.2006.10096.x
-
[36]
S., Wang, L., Tanikawa, A., Hirai, Y., & Saitoh, T
Fujii, M. S., Wang, L., Tanikawa, A., Hirai, Y., & Saitoh, T. R. 2024, Science, 384, 1488, doi: 10.1126/science.adi4211
-
[37]
2023, PhRvD, 108, 024042, doi: 10.1103/PhysRevD.108.024042
Gerosa, D., Fumagalli, G., Mould, M., et al. 2023, PhRvD, 108, 024042, doi: 10.1103/PhysRevD.108.024042
-
[39]
2016, Physical Review D, 93, doi: 10.1103/physrevd.93.124066
Gerosa, D., & Kesden, M. 2016, Physical Review D, 93, doi: 10.1103/physrevd.93.124066
-
[40]
Giersz, M., & Heggie, D. C. 1996, MNRAS, 279, 1037, doi: 10.1093/mnras/279.3.1037
-
[41]
2015, MNRAS, 454, 3150, doi: 10.1093/mnras/stv2162
Giersz, M., Leigh, N., Hypki, A., Lützgendorf, N., & Askar, A. 2015, MNRAS, 454, 3150, doi: 10.1093/mnras/stv2162
-
[42]
Ginat, Y. B., & Perets, H. B. 2024, MNRAS, 531, 739, doi: 10.1093/mnras/stae1241
-
[43]
Glebbeek, E., Gaburov, E., de Mink, S. E., Pols, O. R., & Portegies Zwart, S. F. 2009, A&A, 497, 255, doi: 10.1051/0004-6361/200810425
-
[44]
Gontcharov, G. A., Khovritchev, M. Y., Mosenkov, A. V., et al. 2023, MNRAS, 518, 3036, doi: 10.1093/mnras/stac3300 González, E., Kremer, K., Chatterjee, S., et al. 2021, ApJL, 908, L29, doi: 10.3847/2041-8213/abdf5b González, J. A., Sperhake, U., Brügmann, B., Hannam, M., & Husa, S. 2007, Phys. Rev. Lett., 98, 091101, doi: 10.1103/PhysRevLett.98.091101 Go...
-
[45]
1993, ApJ, 403, 271, doi: 10.1086/172200
Goodman, J., & Hut, P. 1993, ApJ, 403, 271, doi: 10.1086/172200
-
[46]
Greene, J. E., Labbe, I., Goulding, A. D., et al. 2024, ApJ, 964, 39, doi: 10.3847/1538-4357/ad1e5f Gültekin, K., Miller, M. C., & Hamilton, D. P. 2004, ApJ, 616, 221, doi: 10.1086/424809 Häberle, M., Neumayer, N., Seth, A., et al. 2024, Nature, 631, 285, doi: 10.1038/s41586-024-07511-z 20
-
[47]
Haehnelt, M. G., Natarajan, P., & Rees, M. J. 1998, Monthly Notices of the Royal Astronomical Society, 300, 817827, doi: 10.1046/j.1365-8711.1998.01951.x Haemmerlé, L., Mayer, L., Klessen, R. S., et al. 2020, SSRv, 216, 48, doi: 10.1007/s11214-020-00673-y
-
[48]
Harris, C. R., Millman, K. J., van der Walt, S. J., et al. 2020, Nature, 585, 357, doi: 10.1038/s41586-020-2649-2
-
[49]
Heggie, D. C. 1975, MNRAS, 173, 729, doi: 10.1093/mnras/173.3.729
-
[50]
Hills, J. G. 1975, AJ, 80, 809, doi: 10.1086/111815
-
[51]
Hills, J. G. 1990, AJ, 99, 979, doi: 10.1086/115388
-
[52]
2017, National Science Review, 4, 685, doi: 10.1093/nsr/nwx116
Hu, W.-R., & Wu, Y.-L. 2017, National Science Review, 4, 685, doi: 10.1093/nsr/nwx116
-
[53]
Hughes, S. A., & Blandford, R. D. 2003, ApJL, 585, L101, doi: 10.1086/375495
-
[54]
Hunter, J. D. 2007, Computing in Science and Engineering, 9, 90, doi: 10.1109/MCSE.2007.55
-
[55]
Hurley, J. R., Pols, O. R., & Tout, C. A. 2000, MNRAS, 315, 543, doi: 10.1046/j.1365-8711.2000.03426.x
-
[56]
Monthly Notices of the Royal Astronomical Society , author =
Hurley, J. R., Tout, C. A., & Pols, O. R. 2002, MNRAS, 329, 897, doi: 10.1046/j.1365-8711.2002.05038.x
-
[57]
Inayoshi, K., Haiman, Z., & Ostriker, J. P. 2016, MNRAS, 459, 3738, doi: 10.1093/mnras/stw836
-
[58]
doi:10.1146/annurev-astro-120419-014455 , eprint =
Inayoshi, K., Visbal, E., & Haiman, Z. 2020, ARA&A, 58, 27, doi: 10.1146/annurev-astro-120419-014455
-
[59]
2020, PASJ, 72, 13, doi: 10.1093/pasj/psz133
Iwasawa, M., Namekata, D., Nitadori, K., et al. 2020, PASJ, 72, 13, doi: 10.1093/pasj/psz133
-
[60]
2016, PASJ, 68, 54, doi: 10.1093/pasj/psw053
Iwasawa, M., Tanikawa, A., Hosono, N., et al. 2016, PASJ, 68, 54, doi: 10.1093/pasj/psw053
-
[61]
Jiang, Y.-F., Stone, J. M., & Davis, S. W. 2014, ApJ, 796, 106, doi: 10.1088/0004-637X/796/2/106
-
[62]
Jiang, Y.-F., Stone, J. M., & Davis, S. W. 2019, ApJ, 880, 67, doi: 10.3847/1538-4357/ab29ff
-
[63]
Johnson, J. L., & Bromm, V. 2007, MNRAS, 374, 1557, doi: 10.1111/j.1365-2966.2006.11275.x
-
[64]
Kashikawa, N., Ishizaki, Y., Willott, C. J., et al. 2015, ApJ, 798, 28, doi: 10.1088/0004-637X/798/1/28
-
[65]
Kidder, L. E. 1995, Phys. Rev. D, 52, 821, doi: 10.1103/PhysRevD.52.821
-
[66]
2023, ApJL, 957, L7, doi: 10.3847/2041-8213/ad037a
Kokorev, V., Fujimoto, S., Labbe, I., et al. 2023, ApJL, 957, L7, doi: 10.3847/2041-8213/ad037a
-
[67]
2017, Scientific Reports, 7, 41617, doi: 10.1038/srep41617
Guo, J.-K. 2017, Scientific Reports, 7, 41617, doi: 10.1038/srep41617
-
[68]
Kormendy, J., & McClure, R. D. 1993, AJ, 105, 1793, doi: 10.1086/116555
-
[69]
Kroupa, P. 2001, MNRAS, 322, 231, doi: 10.1046/j.1365-8711.2001.04022.x
-
[70]
2020, MNRAS, 498, 5652, doi: 10.1093/mnras/staa2276
Kroupa, P., Subr, L., Jerabkova, T., & Wang, L. 2020, MNRAS, 498, 5652, doi: 10.1093/mnras/staa2276
-
[71]
A., Omukai, K., Habouzit, M., Schleicher, D
Latif, M. A., Omukai, K., Habouzit, M., Schleicher, D. R. G., & Volonteri, M. 2016, ApJ, 823, 40, doi: 10.3847/0004-637X/823/1/40
-
[72]
2014, ApJ, 792, 78, doi: 10.1088/0004-637X/792/1/78
Spaans, M. 2014, ApJ, 792, 78, doi: 10.1088/0004-637X/792/1/78
-
[73]
Niemeyer, J. C. 2013, MNRAS, 436, 2989, doi: 10.1093/mnras/stt1786
-
[74]
Lee, H. M. 1987, ApJ, 319, 801, doi: 10.1086/165498
-
[75]
Lousto, C. O., & Zlochower, Y. 2008, Phys. Rev. D, 77, 044028, doi: 10.1103/PhysRevD.77.044028
-
[76]
Lousto, C. O., & Zlochower, Y. 2013, Phys. Rev. D, 87, 084027, doi: 10.1103/PhysRevD.87.084027
-
[77]
O., Zlochower, Y., Dotti, M., & Volonteri, M
Lousto, C. O., Zlochower, Y., Dotti, M., & Volonteri, M. 2012, Physical Review D, 85, 084015. https://api.semanticscholar.org/CorpusID:88504052
2012
-
[78]
2016, Classical and Quantum Gravity, 33, 035010, doi: 10.1088/0264-9381/33/3/035010
Luo, J., Chen, L.-S., Duan, H.-Z., et al. 2016, Classical and Quantum Gravity, 33, 035010, doi: 10.1088/0264-9381/33/3/035010
-
[79]
2020, Classical and Quantum Gravity, 37, 185013, doi: 10.1088/1361-6382/aba66a
Luo, J., Bai, Y.-Z., Cai, L., et al. 2020, Classical and Quantum Gravity, 37, 185013, doi: 10.1088/1361-6382/aba66a
-
[80]
Mandel, I. 2007, arXiv e-prints, arXiv:0707.0711, doi: 10.48550/arXiv.0707.0711
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.0707.0711 2007
-
[81]
2016, MNRAS, 459, 3432, doi: 10.1093/mnras/stw869
Mapelli, M. 2016, MNRAS, 459, 3432, doi: 10.1093/mnras/stw869
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.