Nuclear coups: dynamics of black holes in galaxy mergers
  We study the dynamical evolution of supermassive black holes (BHs) in merging
galaxies on scales of hundreds of kpc to 10 pc, to identify the physical
processes that aid or hinder the orbital decay of BHs. We present
hydrodynamical simulations of galaxy mergers with a resolution of $\leq$20 pc,
chosen 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, 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 that of the
larger galaxy (`nuclear coup'), as star formation grows a denser nuclear cusp
in the smaller galaxy. 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 the secondary's nucleus is
disrupted, as the infalling BH is more massive, and it also finds itself in a
denser stellar environment.
