RABBITS -- I. The crucial role of nuclear star formation in driving the coalescence of supermassive black hole binaries

Abstract

In this study of the `Resolving supermAssive Black hole Binaries In galacTic hydrodynamical Simulations' (RABBITS) series, we focus on the hardening and coalescing process of supermassive black hole (SMBH) binaries in galaxy mergers. For simulations including different galaxy formation processes (i.e. gas cooling, star formation, SMBH accretion, stellar and AGN feedback), we systematically control the effect of stochastic eccentricity by fixing it to similar values during the SMBH hardening phase. We find a strong correlation between the SMBH merger time-scales and the presence of nuclear star formation. Throughout the galaxy merging process, gas condenses at the centre due to cooling and tidal torques, leading to nuclear star formation. These recently formed stars, which inherit low angular momenta from the gas, contribute to the loss cone and assist in the SMBH hardening via three-body interactions. Compared to non-radiative hydrodynamical runs, the SMBH merger time-scales measured from the runs including cooling, stellar and SMBH physical processes tend to be shortened by a factor of $\sim 1.7$. After fixing the eccentricity to the range of $e\sim 0.6$--$0.8$ during the hardening phase, the simulations with AGN feedback reveal merger time-scales of $\sim 100$--$500$ Myr for disc mergers and $\sim 1$--$2$ Gyr for elliptical mergers. With a semi-analytical approach, we find that the torque interaction between the binary and its circumbinary disc has minimal impact on the shrinking of the binary orbit in our retrograde galaxy merger. Our results are useful in improving the modelling of SMBH merger time-scales and gravitational wave event rates.

18 pages, 8 main + 3 appendix figures, renamed as RABBITS I, accepted for publication in MNRAS

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• RABBITS -- II. The impact of AGN feedback on coalescing supermassive black holes in disc and elliptical galaxy mergers
• Identifying supermassive black hole recoil in elliptical galaxies
• Tracking on-the-fly massive black hole binary evolution and coalescence in galaxy simulations: RAMCOAL
• First Light And Reionization Epoch Simulations (FLARES) -- XIX: Supermassive black hole mergers in the early Universe and their environmental dependence
• Dynamical evolution timescales for the supermassive black hole system in the galaxy NGC 7727 (Arp 222)
• Rapid sinking and efficient mergers of supermassive black holes in compact high-redshift galaxies