Baryons, Dark Matter, and the Jeans Mass in Simulations of Cosmological Structure Formation

Simulations incorporating hydrodynamics are becoming increasingly important tools for studying the formation and evolution of cosmological structure. We investigate the properties of such hybrid gravitational/hydrodynamical simulations, examining both the numerics as well as the general physical properties of gravitationally driven, hierarchical collapse in a mixed baryonic/dark matter fluid. We demonstrate that, under certain restrictions, such simulations reliably converge with increasing resolution to a consistent solution. The dark matter achieves convergence provided that the relevant scales dominating nonlinear collapse are resolved. If the gas has a minimum temperature (as expected, for example, when intergalactic gas is heated by photoionization due to the ultraviolet background) and the corresponding Jeans mass is resolved, then the baryons also converge to a consistent distribution. However, if there is no minimum baryonic collapse mass (or if this scale is not resolved), then the baryon results err in a systematic