Tuesdays 10:30 - 11:30 | Fridays 11:30 - 12:30
Showing votes from 2017-02-21 11:30 to 2017-02-24 12:30 | Next meeting is Friday May 22nd, 11:30 am.
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.
Assuming the existence of a local, analytic, unitary UV completion in a Poincar\'{e} invariant scalar field theory with a mass gap, we derive an infinite number of positivity requirements using the known properties of the amplitude at and away from the forward scattering limit. These take the form of bounds on combinations of the pole subtracted scattering amplitude and its derivatives. In turn, these positivity requirements act as constraints on the operator coefficients in the low energy effective theory. For certain theories these constraints can be used to place an upper bound on the mass of the next lightest state that must lie beyond the low energy effective theory if such a UV completion is to ever exist.