FIRE-2 Simulations: Physics versus Numerics in Galaxy Formation

Philip F Hopkins, Andrew Wetzel, Dusan Keres, Claude-Andre Faucher-Giguere, Eliot Quataert, Michael Boylan-Kolchin, Norman Murray, Christopher C. Hayward, Shea Garrison-Kimmel, Cameron Hummels, Robert Feldmann, Paul Torrey, Xiangcheng Ma, Daniel Angles-Alcazar, Kung-Yi Su, Matthew Orr, Denise Schmitz, Ivanna Escala, Robyn Sanderson, Michael Y. Grudic, Zachary Hafen, Ji-Hoon Kim, Alex Fitts, James S. Bullock, Coral Wheeler, T. K. Chan, Oliver D. Elbert, Desika Narananan

Published 2017-02-20Version 1

The Feedback In Realistic Environments (FIRE) project explores the role of feedback in cosmological simulations of galaxy formation. Previous FIRE simulations used an identical source code (FIRE-1) for consistency. Now, motivated by the development of more accurate numerics (hydrodynamic solvers, gravitational softening, supernova coupling) and the exploration of new physics (e.g. magnetic fields), we introduce FIRE-2, an updated numerical implementation of FIRE physics for the GIZMO code. We run a suite of simulations and show FIRE-2 improvements do not qualitatively change galaxy-scale properties relative to FIRE-1. We then pursue an extensive study of numerics versus physics in galaxy simulations. Details of the star-formation (SF) algorithm, cooling physics, and chemistry have weak effects, provided that we include metal-line cooling and SF occurs at higher-than-mean densities. We present several new resolution criteria for high-resolution galaxy simulations. Most galaxy-scale properties are remarkably robust to the numerics that we test, provided that: (1) Toomre masses (cold disk scale heights) are resolved; (2) feedback coupling ensures conservation and isotropy, and (3) individual supernovae are time-resolved. As resolution increases, stellar masses and profiles converge first, followed by metal abundances and visual morphologies, then properties of winds and the circumgalactic medium. The central (~kpc) mass concentration of massive (L*) galaxies is sensitive to numerics, particularly how winds ejected into hot halos are trapped, mixed, and recycled into the galaxy. Multiple feedback mechanisms are required to reproduce observations: SNe regulate stellar masses; OB/AGB mass loss fuels late-time SF; radiative feedback suppresses instantaneous SFRs and accretion onto dwarfs. We provide tables, initial conditions, and the numerical algorithms required to reproduce our simulations.