SPHGal: smoothed particle hydrodynamics with improved accuracy for galaxy simulations

We present the smoothed particle hydrodynamics (SPH) implementation SPHGal, which combines some recently proposed improvements in GADGET. This includes a pressure-entropy formulation with a Wendland kernel, a higher order estimate of velocity gradients, a modified artificial viscosity switch with a modified strong limiter, and artificial conduction of thermal energy. With a series of idealized hydrodynamic tests, we show that the pressure-entropy formulation is ideal for resolving fluid mixing at contact discontinuities but performs conspicuously worse at strong shocks due to the large entropy discontinuities. Including artificial conduction at shocks greatly improves the results. In simulations of Milky Way like disc galaxies a feedback-induced instability develops if too much artificial viscosity is introduced. Our modified artificial viscosity scheme prevents this instability and shows efficient shock capturing capability. We also investigate the star formation rate and the galactic outflow. The star formation rates vary slightly for different SPH schemes while the mass loading is sensitive to the SPH scheme and significantly reduced in our favoured implementation. We compare the accretion behaviour of the hot halo gas. The formation of cold blobs, an artefact of simple SPH implementations, can be eliminated efficiently with proper fluid mixing, either by conduction and/or by using a pressure-entropy formulation.