Repeated mergers and ejection of black holes within nuclear star clusters

Giacomo Fragione*, Joseph Silk

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


Current stellar evolution models predict a dearth of black holes (BHs) with masses 50 M and 5 M, and intermediate-mass black holes (IMBHs; ∼ 102-105 M) have not yet been detected beyond any reasonable doubt. A natural way to form massive BHs is through repeated mergers, detectable via gravitational wave emission with current LIGO/Virgo or future LISA and ET observations. Nuclear star clusters (NSCs) have masses and densities high enough to retain most of the merger products, which acquire a recoil kick at the moment of merger. We explore the possibility that IMBHs may be born as a result of repeated mergers in NSCs, and show how their formation pathways depend on the NSC mass and density, and BH spin distribution. We find that BHs in the pair-instability mass gap can be formed and observed by LIGO/Virgo, and show that the typical mass of the ejected massive BHs is 400-500 M, with velocities of up to a few thousand km s−1. Eventually, some of these IMBHs can become the seeds of supermassive BHs, observed today in the centres of galaxies. In dwarf galaxies, they could potentially solve the abundance, core-cusp, too-big-to-fail, ultra-faint, and baryon-fraction issues via plausible feedback scenarios.

Original languageEnglish (US)
Pages (from-to)4591-4604
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Nov 1 2020


  • Dynamics
  • Galaxies: dwarf
  • Galaxies: kinematics
  • Galaxy: centre
  • Stars: kinematics and dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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