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

Abstract

ABSTRACT Cosmic rays (CRs) are dynamically important in the evolution of galaxies by regulating star formation and powering galactic outflows. However, to what extent CRs regulate galaxy formation depends on the coupling strength of CRs with the ambient plasma and the effective CR transport speed. Moreover, both properties sensitively 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 1010 to $10^{12}\, \mathrm{M}_\odot$ and compare dynamically coupled CRs in the grey approximation with a spectrally resolved model that includes CR momenta from $0.1\, \mathrm{GeV} c^{-1}$ to $100\, \mathrm{TeV} c^{-1}$. We find that hadronic cooling of CRs dominates over Alfvén cooling, with the latter emulating CR losses as a result of streaming of CRs down their pressure gradient. While star formation rates and galaxy morphologies are only mildly affected by the spectral CR modelling, mass loading factors of galactic outflows can differ by up to a factor of 4 in dwarf galaxies. All simulated low-mass haloes (M = 1010, 1011, and $3\times 10^{11}\, \mathrm{M}_\odot$) drive strong outflows, where CR transport is temporally dominated by advection. In contrast, the Milky Way-mass galaxy with $M=10^{12}\, \mathrm{M}_\odot$ does not drive sustained outflows, so that CR transport is entirely dominated by diffusion. The effective energy weighted diffusion coefficients vary by two orders of magnitude from the canonical energy-weighted values of $\langle {D}\rangle _{e_\mathrm{cr}}\sim 10^{28}\, \mathrm{cm^2\, s^{-1}}$ in the disc up to $3\times 10^{29}\, \mathrm{cm^2\, s^{-1}}$ in the circumgalactic medium, where we observe substantial temperature and CR pressure differences between our grey and spectral CR models.