Abstract
Abstract Recently, the advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) has detected black hole (BH) merger events, most of which are sourced by BHs more massive than 30 M ⊙. Especially, the observation of GW170104 suggests dynamically assembled binaries favoring a distribution of misaligned spins. It has been argued that mergers of unassociated BHs can be engendered through a chance meeting in a multiple BH system under gas-rich environments. In this paper, we consider the merger of unassociated BHs, concordant with the massive BH merger events. To that end, we simulate a multiple BH system with a post-Newtonian N-body code incorporating gas accretion and general relativistic effects. As a result, we find that gas dynamical friction effectively promotes a three-body interaction of BHs in dense gas of n gas ≳ 106 cm−3, so that BH mergers can take place within 30 Myr. This scenario predicts an isotropic distribution of spin tilts. In the concordant models with GW150914, the masses of seed BHs are required to be ≳25 M ⊙. The potential sites of such chance meeting BH mergers are active galactic nucleus (AGN) disks and dense interstellar clouds. Assuming the LIGO O1, we roughly estimate the event rates for PopI BHs and PopIII BHs in AGN disks to be ≃1–2 yr−1 and ≃1 yr−1, respectively. Multiple episodes of AGNs may enhance the rates by roughly an order of magnitude. For massive PopI BHs in dense interstellar clouds the rate is ≃0.02 yr−1. Hence, high-density AGN disks are a more plausible site for mergers of chance meeting BHs.
Highlights
INTRODUCTIONGravitational wave (GW) emission associated with black hole (BH) mergers has been detected by advanced LIGO (aLIGO), in the events of GW150914 (Abbott et al 2016a), GW151226 (Abbott et al 2016b), GW170104 (Abbott et al 2017a), GW170608 (Abbott et al 2017b), and GW170814 (Abbott et al 2017c)
We find that gas dynamical friction effectively promotes threebody interaction of black hole (BH) in dense gas of ngas 106 cm−3, so that BH mergers can take place within 30 Myr
Gravitational wave (GW) emission associated with black hole (BH) mergers has been detected by advanced LIGO, in the events of GW150914 (Abbott et al 2016a), GW151226 (Abbott et al 2016b), GW170104 (Abbott et al 2017a), GW170608 (Abbott et al 2017b), and GW170814 (Abbott et al 2017c)
Summary
Gravitational wave (GW) emission associated with black hole (BH) mergers has been detected by advanced LIGO (aLIGO), in the events of GW150914 (Abbott et al 2016a), GW151226 (Abbott et al 2016b), GW170104 (Abbott et al 2017a), GW170608 (Abbott et al 2017b), and GW170814 (Abbott et al 2017c). Tagawa et al (2015, 2016) have proposed mergers of unassociated BHs through a ”chance meeting” in gas-rich environments, without making a priori assumption of a BH binary. They have demonstrated that a multiple stellar-mass BH system embedded in dense gas can engender mergers of BHs through gas dynamical friction and three-body interaction, which predicts an isotropic distribution of spin tilts. Simulations are performed with a highly accurate post-Newtonian N -body code, where such general relativistic effects as the pericenter shift and GW emission are taken into consideration In these simulations, the effects of gas dynamical friction and Hoyle-Lyttleton mass accretion by ambient gas are incorporated. We roughly estimate the event rates of such BH mergers both in galactic centers and in dense interstellar clouds
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
