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 and stellar radiation. 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
$10^{10}$, $10^{11}$ and $10^{12}\, \rm M_{\odot}$ with a maximum resolution of
$9 \,\rm pc$. We find that CRs reduce star formation rate in our two dwarf
galaxies by a factor 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 $\kappa = 10^{27}$ to $\rm 3\times 10^{29}\,
cm^2\, s^{-1}$. With lower $\kappa$, CRs remain confined for longer on small
scales and are consequently efficient in suppressing star formation, whereas a
higher diffusion coefficient reduces the effect on star formation and increases
the generation of cold outflows. Finally, we compare CR feedback to a
calibrated 'strong' supernova feedback model known to sufficiently regulate
star formation in high-redshift cosmological simulations. We find that CR
feedback is not sufficiently strong to replace this strong supernova 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.
