Radiation-magnetohydrodynamics simulations of cosmic ray feedback in disc galaxies

Cosmic rays (CRs) are thought to play an important role in galaxy evolution. We study their effect when coupled to other important sources of feedback, namely supernovae (SNe) and stellar radiation, by including CR anisotropic diffusion and radiative losses but neglecting CR streaming. Using the RAMSES-RT code, we perform the first radiation-magnetohydrodynamics simulations of isolated disc galaxies with and without CRs. We study galaxies embedded in dark matter haloes of 10¹⁰, 10¹¹, and $10^{12}\, \rm M_{\odot }$ with a maximum resolution of $9 \, \rm pc$. We find that CRs reduce the star formation (SF) rate in our two dwarf galaxies by a factor of 2, with decreasing efficiency with increasing galaxy mass. They increase significantly the outflow mass loading factor in all our galaxies and make the outflows colder. We study the impact of the CR diffusion coefficient, exploring values from κ = 10²⁷ to $\rm 3\times 10^{29}\, cm^2\, s^{-1}$. With a lower κ, CRs remain confined for longer on small scales and are consequently efficient in suppressing SF, whereas a higher diffusion coefficient reduces the effect on SF and increases the generation of cold outflows. Finally, we compare CR feedback to a calibrated 'strong' SN feedback model known to sufficiently regulate SF in high-redshift cosmological simulations. We find that CR feedback is not sufficiently strong to replace this strong SN feedback. As they tend to smooth out the ISM and fill it with denser gas, CRs also lower the escape fraction of Lyman continuum photons from galaxies.