We study the dynamical evolution of supermassive black holes (BHs) in merging galaxies on scales of hundreds of kpc to 10 pc, to highlight and identify the physical processes that aid or hinder the orbital decay of BHs down to pc scale. We present hydrodynamical simulations of unequal-mass galaxy mergers ($G_1$ and $G_2$ are the larger and smaller galaxies, respectively), with a variety of orbital configurations, that bridge the gap between large-scale, low-resolution merger simulations and the small-scale, high-resolution simulations of BH-binary evolution. Our simulations resolve $<20$-pc scales in order to accurately track the motion of the nuclei and provide a realistic environment for the evolution of the BHs. We find that, during the late stages of the merger, tidal shocks inject energy in the nuclei ($N_1$ and $N_2$), causing one or both nuclei to be disrupted and leaving their BH ‘naked’, without any bound gas or stars. In many cases, the nucleus that is ultimately disrupted is $N_1$ (‘nuclear coup’), as nuclear star formation causes the secondary nucleus ($N_2$) to grow a denser nuclear cusp. This happens because tidal torques mostly affect $G_2$, causing gas inflows that facilitate nuclear star formation. As the mass ratio of the merger decreases, however, $G_2$ gets more strongly stripped, preventing it from efficiently forming stars and building a dense enough central cusp to have a nuclear coup. We supplement our simulations with an analytical estimate of the orbital decay time required for the BHs to form a binary at unresolved scales, due to dynamical friction. We find that, when a nuclear coup occurs, the time-scale is much shorter than when $N_2$ is disrupted, as the infalling BH is more massive, and it also finds itself in a denser stellar environment.
Date added: Wed, 30 Oct 13