Astronomy Colloquium

As already established in Frenk et al. 1999, entropy profiles of galaxy clusters are systematically different when simulated with SPH versus Eulerian mesh codes. This systematic difference has persisted until now, casting doubts on the ability of hydrodynamical schemes to follow complex hydrodynamical flows in full cosmological settings reliably. I will present a detailed comparison between the well-known SPH code GADGET and the new moving-mesh code AREPO on a number of hydrodynamical test problems. Through a variety of numerical experiments with increasing complexity I will establish a clear link between simple test problems with known analytic solutions and systematic numerical effects seen in cosmological simulations of galaxy formation. These tests demonstrate deficiencies of the SPH method in several sectors. An inadequate treatment of fluid instabilities in GADGET suppresses entropy generation by mixing, underestimates vorticity generation in curved shocks and prevents efficient gas stripping from infalling substructures. These accuracy problems not only manifest themselves in idealized hydrodynamical tests, but also propagate to more realistic simulation setups of galaxy formation, ultimately affecting local and global gas properties in the full cosmological framework.