The importance of black hole repositioning for galaxy formation
  simulations
  Active galactic nucleus (AGN) feedback from accreting supermassive black
holes (SMBHs) is an essential ingredient of galaxy formation simulations. The
orbital evolution of SMBHs is affected by dynamical friction that cannot be
predicted self-consistently by contemporary simulations of galaxy formation in
representative volumes. Instead, such simulations typically use a simple
"repositioning" of SMBHs, but the effects of this approach on SMBH and galaxy
properties have not yet been investigated systematically. Based on a suite of
smoothed particle hydrodynamics simulations with the SWIFT code and a
Bondi-Hoyle-Lyttleton subgrid gas accretion model, we investigate the impact of
repositioning on SMBH growth and on other baryonic components through AGN
feedback. Across at least a factor ~1000 in mass resolution, SMBH repositioning
(or an equivalent approach) is a necessary prerequisite for AGN feedback;
without it, black hole growth is negligible. Limiting the effective
repositioning speed to $\lesssim$ 10 km/s delays the onset of AGN feedback and
severely limits its impact on stellar mass growth in the centre of massive
galaxies. Repositioning has three direct physical consequences. It promotes
SMBH mergers and thus accelerates their initial growth. In addition, it raises
the peak density of the ambient gas and reduces the SMBH velocity relative to
it, giving a combined boost to the accretion rate that can reach many orders of
magnitude. Our results suggest that a more sophisticated and/or better
calibrated treatment of SMBH repositioning is a critical step towards more
predictive galaxy formation simulations.
