But What About… Cosmic Rays, Magnetic Fields, Conduction, & Viscosity in Galaxy Formation

Hopkins et al, available on arXiv

Abstract: We present a suite of high-resolution cosmological simulations, using the FIRE-2 feedback physics together with explicit treatment of magnetic fields, anisotropic conduction and viscosity, and cosmic rays (CRs) injected by supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultra-faint dwarf (Mstar~10^4 Msun, Mhalo~10^9 Msun) through Milky Way masses, systematically vary CR parameters (e.g. the diffusion coefficient kappa and streaming velocity), and study an ensemble of galaxy properties (masses, star formation histories, mass profiles, phase structure, morphologies). We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved (>~1 pc) scales have small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs (Mstar<<10^10 Msun, Mhalo<~10^11 Msun), or at high redshifts (z>~1-2), for any physically-reasonable parameters. However at higher masses (Mhalo>~10^11 Msun) and z<~1-2, CRs can suppress star formation by factors ~2-4, given relatively high effective diffusion coefficients kappa>~3×10^29 cm^2 s^-1. At lower kappa, CRs take too long to escape dense star-forming gas and lose energy to hadronic collisions, producing negligible effects on galaxies and violating empirical constraints from gamma-ray emission. But around kappa~3×10^29 cm^2 s^-1, CRs escape the galaxy and build up a CR-pressure-dominated halo which supports dense, cool (T<<10^6 K) gas that would otherwise rain onto the galaxy. CR heating (from collisional and streaming losses) is never dominant.