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Active galactic nucleus (AGN) feedback from accreting supermassive blackholes (SMBHs) is an essential ingredient of galaxy formation simulations. Theorbital evolution of SMBHs is affected by dynamical friction that cannot bepredicted self-consistently by contemporary simulations of galaxy formation inrepresentative volumes. Instead, such simulations typically use a simple"repositioning" of SMBHs, but the effects of this approach on SMBH and galaxyproperties have not yet been investigated systematically. Based on a suite ofsmoothed particle hydrodynamics simulations with the SWIFT code and aBondi-Hoyle-Lyttleton subgrid gas accretion model, we investigate the impact ofrepositioning on SMBH growth and on other baryonic components through AGNfeedback. 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 effectiverepositioning speed to $\lesssim$ 10 km/s delays the onset of AGN feedback andseverely limits its impact on stellar mass growth in the centre of massivegalaxies. Repositioning has three direct physical consequences. It promotesSMBH mergers and thus accelerates their initial growth. In addition, it raisesthe peak density of the ambient gas and reduces the SMBH velocity relative toit, giving a combined boost to the accretion rate that can reach many orders ofmagnitude. Our results suggest that a more sophisticated and/or bettercalibrated treatment of SMBH repositioning is a critical step towards morepredictive galaxy formation simulations.

The importance of black hole repositioning for galaxy formation simulations