The birth of a supermassive black hole binary
  We study the dynamical evolution of supermassive black holes, in the late
stage of galaxy mergers, from kpc to pc scales. In particular, we capture the
formation of the binary, a necessary step before the final coalescence, and
trace back the main processes causing the decay of the orbit. We use
hydrodynamical simulations of galaxy mergers with different resolutions, from
$20\,\rm pc$ down to $1\,\rm pc$, in order to study the effects of the
resolution on our results, remove numerical effects, and assess that resolving
the influence radius of the orbiting black hole is a minimum condition to fully
capture the formation of the binary. Our simulations include the relevant
physical processes, namely star formation, supernova feedback, accretion onto
the black holes and the ensuing feedback. We find that, in these mergers,
dynamical friction from the smooth stellar component of the nucleus is the main
process that drives black holes from kpc to pc scales. Gas does not play a
crucial role and even clumps do not induce scattering or perturb the orbits. We
compare the time needed for the formation of the binary to analytical
predictions and suggest how to apply such analytical formalism to obtain
estimates of binary formation times in lower resolution simulations.
