Hierarchical black hole mergers in young, globular and nuclear star clusters: the effect of metallicity, spin and cluster properties
Abstract
We explore hierarchical black hole (BH) mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs), accounting for both original and dynamically assembled binary BHs (BBHs). We find that the median mass of both first and nthgeneration dynamical mergers is larger in GCs and YSCs with respect to NSCs because the lighter BHs are ejected by supernova kicks from the lower mass clusters. Also, first and nthgeneration BH masses are strongly affected by the metallicity of the progenitor stars: the median mass of the primary BH of a nthgeneration merger is ~2438 M_{⊙} (~915 M_{⊙}) in metalpoor (metalrich) NSCs. The maximum BH mass mainly depends on the escape velocity: BHs with mass up to several thousand M_{⊙} form in NSCs, while YSCs and GCs host BHs with mass up to several hundred M_{⊙}. Furthermore, we calculate the fraction of mergers with at least one component in the pairinstability mass gap (f_{PI}) and in the intermediatemass BH regime (f_{IMBH}). In the fiducial model for dynamical BBHs with metallicity Z = 0.002, we find f_{PI} ≈ 0.05, 0.02 and 0.007 (f_{IMBH} ≈ 0.01, 0.002 and 0.001) in NSCs, GCs and YSCs, respectively. Both f_{PI} and f_{IMBH} drop by at least one order of magnitude at solar metallicity. Finally, we investigate the formation of GW190521 by assuming that it is either a nearly equalmass BBH or an intermediatemass ratio inspiral.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 July 2021
 DOI:
 10.1093/mnras/stab1334
 arXiv:
 arXiv:2103.05016
 Bibcode:
 2021MNRAS.505..339M
 Keywords:

 black hole physics;
 gravitational waves;
 stars: black holes;
 stars: kinematics and dynamics;
 galaxies: star clusters: general;
 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Astrophysics of Galaxies;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 20 pages, 13 figures, accepted for publication in MNRAS