We present direct N -body simulations, carried out with NBODY 6 + + GPU , of young and compact low-metallicity ( Z = 0.0002) star clusters with 1.1 × 10 5 stars, a velocity dispersion of ∼15 km s −1 , a half-mass radius R h = 0.6 pc, and a binary fraction of 10 per cent including updated evolution models for stellar winds and (pulsation) pair-instability supernovae (PSNe). Within the first tens of megayears, each cluster hosts several black hole (BH) merger events which nearly co v er the complete mass range of primary and secondary BH masses for current LIGO-Virgo-KAGRA gravitational wave detections. The importance of gravitational recoil is estimated statistically during post-processing analysis. We present possible formation paths of massive BHs abo v e the assumed lower PSN mass-gap limit (45 M  ) into the intermediate-mass black hole (IMBH) regime ( > 100 M  ) which include collisions of stars, BHs, and the direct collapse of stellar merger remnants with low core masses. The stellar evolution updates result in the early formation of heavier stellar BHs compared to the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star–BH mergers, we present a first-generation formation scenario for GW190521-like events: a merger of two BHs which reached the PSN mass-gap merging with massive stars. This event is independent of gravitational recoil and therefore concei v able in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153, 173 M  ) which is expected to merge within a Hubble time.

Black hole mergers in compact star clusters and massive black hole formation beyond the mass gap

Manuel Arca-Sedda
Writing – Review & Editing
;
2022-01-01

Abstract

We present direct N -body simulations, carried out with NBODY 6 + + GPU , of young and compact low-metallicity ( Z = 0.0002) star clusters with 1.1 × 10 5 stars, a velocity dispersion of ∼15 km s −1 , a half-mass radius R h = 0.6 pc, and a binary fraction of 10 per cent including updated evolution models for stellar winds and (pulsation) pair-instability supernovae (PSNe). Within the first tens of megayears, each cluster hosts several black hole (BH) merger events which nearly co v er the complete mass range of primary and secondary BH masses for current LIGO-Virgo-KAGRA gravitational wave detections. The importance of gravitational recoil is estimated statistically during post-processing analysis. We present possible formation paths of massive BHs abo v e the assumed lower PSN mass-gap limit (45 M  ) into the intermediate-mass black hole (IMBH) regime ( > 100 M  ) which include collisions of stars, BHs, and the direct collapse of stellar merger remnants with low core masses. The stellar evolution updates result in the early formation of heavier stellar BHs compared to the previous model. The resulting higher collision rates with massive stars support the rapid formation of massive BHs. For models assuming a high accretion efficiency for star–BH mergers, we present a first-generation formation scenario for GW190521-like events: a merger of two BHs which reached the PSN mass-gap merging with massive stars. This event is independent of gravitational recoil and therefore concei v able in dense stellar systems with low escape velocities. One simulated cluster even forms an IMBH binary (153, 173 M  ) which is expected to merge within a Hubble time.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12571/31497
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