The Formation of a Milky Way-Sized Disk Galaxy. I. A Comparison of Numerical Methods

Abstract

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. We investigate these two fundamental aspects in a new comparison project using a set of cosmological hydrodynamic simulations of a Milky Way-size galaxy. In this study, we focus on the comparison of two particle-based hydrodynamics methods: an improved smoothed particle hydrodynamics (SPH) code Gadget, and a Lagrangian Meshless Finite-Mass (MFM) code Gizmo. All the simulations in this paper use the same initial conditions and physical models, which include star formation, "energy-driven" outflows, metaldependent cooling, stellar evolution and metal enrichment. 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. Compared to Gizmo, Gadget simulation produced 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 SPH in cosmological hydrodynamic simulations.

STARFORGE: Towards a comprehensive numerical model of star cluster formation and feedback

“…We do note these tests have demonstrated that our default implementation can simultaneously accurately evolve phenomena including gas in regular or warped Keplerian discs, strong interacting shocks, current sheets and flux tubes, supersonic and subsonic turbulence, fluid mixing instabilities (Kelvin-Helmholz, Rayleigh Taylor, etc. ), multifluid dustgas dynamics, collisional+collisionless gravitational dynamics, and reproduces the correct linear growth rates of the magnetorotational instability (MRI) and non-ideal Hall MRI and anisotropic MHD instabilities (magnetothermal, heat-flux-bouyancy) (Hopkins 2015;Hopkins & Raives 2016;Zhu & Li 2016;Lupi, Volonteri & Silk 2017;Deng et al 2019a, b;Moseley et al 2019;Rennehan et al 2019;Hu & Chiang 2020;Panuelos, Wadsley & Kevlahan 2020). Tests of idealized problems involving self-gravitating MHD including the Evrard (1988) problem (spherical collapse of a self-gravitating polytrope), the MHD Zel'dovich (1970) pancake (self-gravitating collapse of an initially linear density perturbation along one dimension in a 3D Hubble flow) demonstrate that the MFM/MFV methods in GIZMO (as well as related moving-mesh methods) converge much more rapidly than popular AMR or SPH methods applied to the same problem (Hopkins 2015;Hopkins & Raives 2016;Hubber, Rosotti & Booth 2018).…”

FIRE-2 simulations: physics versus numerics in galaxy formation

“…Over the mass range simulated (M * ∼ 10 9 − 10 12 M ), the stellar mass functions agree very well at z > 1, but then begin to differ, with SPH tending towards smaller masses by ∼ 0.2 dex by z = 0. Yet another study using still different ISM and feedback models in GIZMO presented in Zhu & Li (2016), who see a similar "divergence" between MFM and some SPH flavors.…”

Gas kinematics, morphology and angular momentum in the FIRE simulations

“…This is critical for studies of angular momentum and rotational support in galaxies, as previous works have found significant spurious angular momentum loss due to numerical effects, particularly at low resolution (Governato et al 2004;Kaufmann et al 2007a). MFM resolves a number of numerical issues that have been shown to cause spurious angular momentum loss in galaxy formation simulations (Zhu & Li 2016;Hopkins et al 2017), including the unphysical cooling of cold low-angular momentum "blobs" from the hot halo into galaxies and numerical torques red between galaxies and their halos (e.g. Okamoto et al 2003;Agertz et al 2007;Torrey et al 2012;Kereš et al 2012;Few et al 2016).…”