Merging of Unequal Mass Binary Black Holes in Non-Axisymmetric Galactic Nuclei

Berczik, Péter and Arca, Sedda Manuel and Sobolenko, Margaryta and Ishchenko, Marina and Sobodar, Olexander (2022) Merging of Unequal Mass Binary Black Holes in Non-Axisymmetric Galactic Nuclei. ASTRONOMY & ASTROPHYSICS, 665. ISSN 0004-6361

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Abstract

In this work we study the stellar-dynamical hardening of unequal mass supermassive black hole (SMBH) binaries in the central regions of galactic nuclei. We present a comprehensive set of direct N-body simulations of the problem, varying both the total mass and the mass ratio of the SMBH binary (SMBHB). Simulations are carried out with the phi-GPU N-body code, which enabled us to fully exploit supercomputers equipped with graphic processing units (GPUs). We adopt initial axisymmetric, rotating models, aimed at reproducing the properties of a galactic nucleus emerging from a galaxy merger event, containing two SMBHs initially unbound. We find no "final-pc problem", as our SMBH's tend to pair and shrink without showing significant signs of stalling. This confirms earlier results and extends them to large particle numbers and for rotating systems. We find that the SMBHB hardening depends on the binary reduced mass ratio via a single parameter function. Our results suggest that, at fixed value of the SMBHB primary mass, the merger time of highly asymmetric binaries is up to four order of magnitudes smaller than equal-mass binaries. This can significantly affect the population of SMBHs potentially detectable as gravitational wave sources.

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