Spectrally resolved cosmic rays - III. Dynamical impactãnd properties of the circumgalactic medium

Abstract

Cosmic rays (CRs)ãre dynamically important in the evolution of galaxies by regulating star formationãnd powering galactic outflo ws. Ho we v er, to what e xtent CRs re gulate galaxy formation depends on the coupling strength of CRs with theãmbient plasmaãnd the ef fecti v e CR transport speed. Moreo v er, both properties sensitiv ely depend on the CR momentum, which is largely unexplored in three-dimensional hydrodynamical simulations. We perform magnetohydrodynamical simulations of entire galaxies with masses ranging from 10 10 to 10 12 M⊙and compare dynamically coupled CRs in the greyãpproximation withã spectrally resolved model that includes CR momenta from 0 . 1 GeV c -1 to 100 TeV c -1 . We find that hadronic cooling of CRs dominates o v er Alfvén cooling, with the latter emulating CR lossesãsã result of streaming of CRs down their pressure gradient. While star formation ratesãnd galaxy morphologiesãre only mildlyãffected by the spectral CR modelling, mass loading factors of galactic outflows can differ by up toã factor of 4 in dwarf galaxies. All simulated low-mass haloes ( M⊙= 10 10 , 10 11 ,ãnd 3 ×10 11 M⊙) drive strong outflows, where CR transport is temporally dominated byãdvection. In contrast, the Milky Way-mass galaxy with M⊙= 10 12 M⊙does not drive sustained outflows, so that CR transport is entirely dominated by diffusion. The ef fecti ve energy weighted diffusion coefficients vary by two orders of magnitude from the canonical energy-weighted values of (D)e cr ∼10 28 cm 2 s -1 in the disc up to 3 ×10 29 cm 2 s -1 in the circumgalactic medium, where we observe substantial temperatureãnd CR pressure differences between our greyãnd spectral CR models.