Comparing AMR and SPH cosmological simulations. I. Dark matter and adiabatic simulations

We compare two cosmological hydrodynamic simulation codes in the context of hierarchical galaxy formation: the Lagrangian smoothed particle hydrodynamics (SPH) code GADGET, and the Eulerian adaptive mesh refinement (AMR) code Enzo. Both codes represent dark matter with the N-body method but use different gravity solvers and fundamentally different approaches for baryonic hydrodynamics. The SPH method in GADGET uses a recently developed entropy conserving'' formulation of SPH, while for the mesh-based Enzo two different formulations of Eulerian hydrodynamics are employed: the piecewise parabolic method (PPM) extended with a dual energy formulation for cosmology, and the artificial viscosity-based scheme used in the magnetohydrodynamics code ZEUS. In this paper we focus on a comparison of cosmological simulations that follow either only dark matter, or also a nonradiative (adiabatic'') hydrodynamic gaseous component. We perform multiple simulations using both codes with varying spatial and mass resolution with identical initial conditions. The dark matter-only runs agree generally quite well provided Enzo is run with a comparatively fine root grid and a low overdensity threshold for mesh refinement, otherwise the abundance of low-mass halos is suppressed. This can be readily understood as a consequence of the hierarchical particle-mesh algorithm used by Enzo to compute gravitational forces, which tends to deliver lower force resolution than the tree-algorithm of GADGET at early times before any adaptive mesh refinement takes place. At comparable force resolution we find that the latter offers substantially better performance and lower memory consumption than the present gravity solver in Enzo. In simulations that include adiabatic gasdynamics we find general agreement in the distribution functions of temperature, entropy, and density for gas of moderate to high overdensity, as found inside dark matter halos. However, there are also some significant differences in the same quantities for gas of lower overdensity. For example, at z = 3 the fraction of cosmic gas that has temperature log T > 0: 5 is similar to 80% for both Enzo ZEUS and GADGET, while it is 40% - 60% for Enzo PPM. We argue that these discrepancies are due to differences in the shock-capturing abilities of the different methods. In particular, we find that the ZEUS implementation of artificial viscosity in Enzo leads to some unphysical heating at early times in preshock regions. While this is apparently a significantly weaker effect in GADGET, its use of an artificial viscosity technique may also make it prone to some excess generation of entropy that should be absent in Enzo PPM. Overall, the hydrodynamical results for GADGET are bracketed by those for Enzo ZEUS and Enzo PPM but are closer to Enzo ZEUS.