The resolution bias: low-resolution feedback simulations are better at destroying galaxies

Feedback from supermassive black holes is thought to play a key role in regulating the growth of host galaxies. Cosmological and galaxy formation simulations using smoothed particle hydrodynamics (SPH), which usually use a fixed mass for SPH particles, often employ the same sub-grid active galactic nuclei (AGN) feedback prescription across a range of resolutions. It is thus important to ask how the impact of the simulated AGN feedback on a galaxy changes when only the numerical resolution (the SPH particle mass) changes. We present a suite of simulations modelling the interaction of an AGN outflow with the ambient turbulent and clumpy interstellar medium in the inner part of the host galaxy at a range of mass resolutions. We find that, with other things being equal, degrading the resolution leads to feedback becoming more efficient at clearing out all gas in its path. For the simulations presented here, the difference in the mass of the gas ejected by AGN feedback varies by more than a factor of 10 between our highest and lowest resolution simulations. This happens because feedback-resistant high-density clumps are washed out at low effective resolutions. We also find that changes in numerical resolution lead to undesirable artefacts in how the AGN feedback affects the AGN immediate environment.