Abstract
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.
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