The dependence of cosmic ray driven galactic winds on halo mass
  Galactic winds regulate star formation in disk galaxies and help to enrich
the circum-galactic medium. They are therefore crucial for galaxy formation,
but their driving mechanism is still poorly understood. Recent studies have
demonstrated that cosmic rays (CRs) can drive outflows if active CR transport
is taken into account. Using hydrodynamical simulations of isolated galaxies
with virial masses between $10^{10}$ and $10^{13}\mathrm{~M_\odot}$, we study
how the properties of CR-driven winds depend on halo mass. CRs are treated in a
two-fluid approximation and their transport is modelled through isotropic or
anisotropic diffusion. We find that CRs are only able to drive mass-loaded
winds beyond the virial radius in haloes with masses below
$10^{12}\mathrm{~M_\odot}$. For our lowest examined halo mass, the wind is
roughly spherical and has velocities of $\sim20\mathrm{~km\;s^{-1}}$. With
increasing halo mass, the wind becomes biconical and can reach ten times higher
velocities. The mass loading factor drops rapidly with virial mass, a
dependence that approximately follows a power-law with a slope between $-1$ and
$-2$. This scaling is slightly steeper than observational inferences, and also
steeper than commonly used prescriptions for wind feedback in cosmological
simulations. The slope is quite robust to variations of the CR injection
efficiency or the CR diffusion coefficient. In contrast to the mass loading,
the energy loading shows no significant dependence on halo mass. While these
scalings are close to successful heuristic models of wind feedback, the
CR-driven winds in our present models are not yet powerful enough to fully
account for the required feedback strength.
