But what about. Cosmic rays, magnetic fields, conduction, and viscosity in galaxy formation

Philip F. Hopkins, T. K. Chan, Shea Garrison-Kimmel, Suoqing Ji, Kung Yi Su, Cameron B. Hummels, Dušan Kereš, Eliot Quataert, Claude André Faucher-Giguère

Research output: Contribution to journalArticlepeer-review

110 Scopus citations


We present and study a large suite of high-resolution cosmological zoom-in simulations, using the FIRE-2 treatment of mechanical and radiative feedback from massive stars, together with explicit treatment of magnetic fields, anisotropic conduction and viscosity (accounting for saturation and limitation by plasma instabilities at high β), and cosmic rays (CRs) injected in supernovae shocks (including anisotropic diffusion, streaming, adiabatic, hadronic and Coulomb losses). We survey systems from ultrafaint dwarf (M∗∼ 104M⊙, Mhalo ∼ 109M⊙) through Milky Way/Local Group (MW/LG) masses, systematically vary uncertain CR parameters (e.g. the diffusion coefficient κand streaming velocity), and study a broad ensemble of galaxy properties [masses, star formation (SF) histories, mass profiles, phase structure, morphologies, etc.]. We confirm previous conclusions that magnetic fields, conduction, and viscosity on resolved (≥ 1 pc) scales have only small effects on bulk galaxy properties. CRs have relatively weak effects on all galaxy properties studied in dwarfs (M∗≪ 1010M⊙, Mhalo ≲1011M⊙), or at high redshifts (z ≳1-2), for any physically reasonable parameters. However, at higher masses (Mhalo ≳ 1011M⊙) and z ≲1-2, CRs can suppress SF and stellar masses by factors ∼2-4, given reasonable injection efficiencies and relatively high effective diffusion coefficients κ≳ 3 × 1029 cm2s-1. At lower κ, CRs take too long to escape dense star-forming gas and lose their energy to collisional hadronic losses, producing negligible effects on galaxies and violating empirical constraints from spallation and γ -ray emission. At much higher κCRs escape too efficiently to have appreciable effects even in the CGM. But around κ∼ 3 × 1029 cm2 s-1, CRs escape the galaxy and build up a CR-pressure-dominated halo which maintains approximate virial equilibrium and supports relatively dense, cool (T≪106 K) gas that would otherwise rain on to the galaxy. CR 'heating' (from collisional and streaming losses) is never dominant.

Original languageEnglish (US)
Pages (from-to)3465-3498
Number of pages34
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Mar 1 2020
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Active
  • Cosmology
  • Evolution
  • Formation
  • Formation
  • Galaxies
  • Galaxies
  • Galaxies
  • Stars
  • Theory


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