Mesh-free hydrodynamics in pkdgrav3 for galaxy formation simulations

We extend the state-of-the-art N-body code pkdgrav3 with the inclusion of mesh-free gas hydrodynamics for cosmological simulations. Two new hydrodynamic solvers have been implemented, the mesh-less finite volume and mesh-less finite mass methods. The solvers manifestly conserve mass, momentum, and energy, and have been validated with a wide range of standard test simulations, including cosmological simulations. We also describe improvements to pkdgrav3 that have been implemented for performing hydrodynamic simulations. These changes have been made with efficiency and modularity in mind, and provide a solid base for the implementation of the required modules for galaxy formation and evolution physics and future porting to GPUs. The code is released in a public repository, together with the documentation, and all the test simulations presented in this work