Spatially offset black holes in the Horizon-AGN simulation and
  comparison to observations
  We study the displacements between the centres of galaxies and their
supermassive black holes (BHs) in the cosmological hydrodynamical simulation
Horizon-AGN, and in a variety of observations from the literature. The BHs in
Horizon-AGN feel a sub-grid dynamical friction force, sourced by the
surrounding gas, which prevents recoiling BHs being ejected from the galaxy. We
find that i) the fraction of spatially offset BHs increases with cosmic time,
ii) BHs live on prograde orbits in the plane of the galaxy with an orbital
radius that decays with time but stalls near $z=0$, and iii) the magnitudes of
offsets from the galaxy centres are substantially larger in the simulation than
in observations. We attribute the stalling of the infall and excessive offset
magnitudes to the fact that dynamical friction from stars and dark matter is
not modelled in the simulation, and hence provide a way to improve the black
hole dynamics of future simulations.
