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

Philip F. Hopkins; T. K. Chan; Shea Garrison-Kimmel; Suoqing Ji; Kung-Yi Su; Cameron B. Hummels; Dusan Keres; Eliot Quataert; Claude-Andre Faucher-Giguere

doi:10.1093/mnras/stz3321

paper

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

arXiv:1905.04321 · doi:10.1093/mnras/stz3321

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 ( $M_{*} \sim 1 0^{4} M_{⊙}$ , $M_{halo} \sim 1 0^{9} M_{⊙}$ ) through Milky Way masses, systematically vary CR parameters (e.g. the diffusion coefficient $κ$ 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 ( $M_{*} ≪ 1 0^{10} M_{⊙}$ , $M_{halo} ≲ 1 0^{11} M_{⊙}$ ), or at high redshifts ( $z ≳ 1 - 2$ ), for any physically-reasonable parameters. However at higher masses ( $M_{halo} ≳ 1 0^{11} M_{⊙}$ ) and $z ≲ 1 - 2$ , CRs can suppress star formation by factors $\sim 2 - 4$ , given relatively high effective diffusion coefficients $κ ≳ 3 \times 1 0^{29} c m^{2} s^{- 1}$ . At lower $κ$ , 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 $γ$ -ray emission. But around $κ \sim 3 \times 1 0^{29} c m^{2} s^{- 1}$ , CRs escape the galaxy and build up a CR-pressure-dominated halo which supports dense, cool ( $T ≪ 1 0^{6}$ K) gas that would otherwise rain onto the galaxy. CR heating (from collisional and streaming losses) is never dominant.

arXiv comment35 pages, 23 figures. Updated to match published (MNRAS) version

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

Abstract

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