The Formation of a Milky Way-sized Disk Galaxy 1. A Comparison of   Numerical Methods
  The long-standing challenge of creating a Milky Way-like disk galaxy from cosmological simulations has motivated significant developments in both numerical methods and physical models in recent years. We investigate these two fundamental aspects in a new comparison project using a set of cosmological hydrodynamic simulations of the formation and evolution of a Milky Way-size galaxy. In this study, we focus on the comparison of two particle-based hydrodynamics methods: the improved smoothed particle hydrodynamics (SPH) code Gadget, and the Lagrangian Meshless Finite-Mass (MFM) code GIZMO. All the simulations in this paper use the same initial conditions and physical models, which include physics of both dark matter and baryons, star formation, "energy-driven" outflow, metal-dependent cooling, stellar evolution and metal enrichment from supernovae. We find that both numerical schemes produce a late-type galaxy with extended gaseous and stellar disks. However, notable differences are present in a wide range of galaxy properties and their evolution, including star formation history, gas content, disk structure and kinematics. In particular, there is significant difference in gas properties and their evolution between the two simulations. Compared to GIZMO, Gadget simulation produces a larger fraction of cold, dense gas at high redshift which fuels rapid star formation and results in a higher stellar mass by $20\%$ and a lower gas fraction by $10\%$ at $z = 0$, and the resulting gas disk is smoother and more coherent in rotation due to damping of turbulent motion by the numerical viscosity in SPH, in contrast to the GIZMO simulation which shows more prominent spiral structure. Given its better convergence properties and lower computational cost, we argue that MFM method is a promising alternative to the widely used SPH in cosmological hydrodynamic simulations. 
Qirong Zhu and Yuexing Li
astro-ph.GA astro-ph.CO