Gravitational Waves and Intermediate-mass Black Hole Retention in Globular Clusters

Giacomo Fragione, Idan Ginsburg, Bence Kocsis

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96 Scopus citations

Abstract

The recent discovery of gravitational waves (GWs) has opened new horizons for physics. Current and upcoming missions, such as LIGO, VIRGO, KAGRA, and LISA, promise to shed light on black holes of every size from stellar mass (SBH) sizes up to supermassive black holes. The intermediate-mass black hole (IMBH) family has not been detected beyond any reasonable doubt. Recent analyses suggest observational evidence for the presence of IMBHs in the centers of two Galactic globular clusters (GCs). In this paper, we investigate the possibility that GCs were born with a central IMBH, which undergoes repeated merger events with SBHs in the cluster core. By means of a semi-analytical method, we follow the evolution of the primordial cluster population in the galactic potential and the mergers of the binary IMBH-SBH systems. Our models predict ≈1000 IMBHs within 1 kpc from the galactic center and show that the IMBH-SBH merger rate density changes from Gpc-3 yr-1 beyond z ≈ 2 to Gpc-3 yr-1 at z ≈ 0. The rates at low redshifts may be significantly higher if young massive star clusters host IMBHs. The merger rates are dominated by IMBHs with masses between 103 and 104 M o. Currently, there are no LIGO/VIRGO upper limits for GW sources in this mass range, but our results show that at design sensitivity, these instruments will detect IMBH-SBH mergers in the coming years. LISA and the Einstein Telescope will be best suited to detect these events. The inspirals of IMBH-SBH systems may also generate an unresolved GW background.

Original languageEnglish (US)
Article number92
JournalAstrophysical Journal
Volume856
Issue number2
DOIs
StatePublished - Apr 1 2018

Funding

We thank Daniel D’Orazio, Oleg Gnedin, and Deirdre Shoemaker for useful discussions and comments. G.F. acknowledges hospitality from the Eötvös Loránd University of Budapest. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 638435 (GalNUC) and by the Hungarian National Research, Development, and Innovation Office grant NKFIH KH-125675 (to B.K.). This work was performed by BK in part at the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607761. I.G. was supported in part by Harvard University and the Institute for Theory and Computation.

Keywords

  • Galaxy: kinematics and dynamics
  • galaxies: star clusters: general
  • stars: black holes
  • stars: kinematics and dynamics

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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