Snap, crackle, pop: sub-grid supernova feedback in AMR simulations of disc galaxies

We compare five sub-grid models for supernova (SN) feedback in adaptive mesh refinement (AMR) simulations of isolated dwarf and L-star disc galaxies with 20-40 pc resolution. The models are thermal dump, stochastic thermal, 'mechanical' (injecting energy or momentum depending on the resolution), kinetic and delayed cooling feedback. We focus on the ability of each model to suppress star formation and generate outflows. Our highest resolution runs marginally resolve the adiabatic phase of the feedback events, which correspond to 40 SN explosions, and the first three models yield nearly identical results, possibly indicating that kinetic and delayed cooling feedback converge to wrong results. At lower resolution all models differ, with thermal dump feedback becoming inefficient. Thermal dump, stochastic and mechanical feedback generate multiphase outflows with mass loading factors β ≪ 1, which is much lower than observed. For the case of stochastic feedback, we compare to published SPH simulations, and find much lower outflow rates. Kinetic feedback yields fast, hot outflows with β ∼ 1, but only if the wind is in effect hydrodynamically decoupled from the disc using a large bubble radius. Delayed cooling generates cold, dense and slow winds with β > 1, but large amounts of gas occupy regions of temperature-density space with short cooling times. We conclude that either our resolution is too low to warrant physically motivated models for SN feedback, that feedback mechanisms other than SNe are important or that other aspects of galaxy evolution, such as star formation, require better treatment.