Cooler and smoother - the impact of cosmic rays on the phase structure of galactic outflows

We investigate the impact of cosmic rays (CRs) on galactic outflows from a multiphase interstellar medium with solar neighbourhood conditions. The three-dimensional magnetohydrodynamical simulations include CRs as a relativistic fluid in the advection-diffusion approximation. The thermal and chemical state of the interstellar medium is computed with a non-equilibrium chemical network. We find that CRs [injected with 10 per cent of the supernova (SN) energy] efficiently support the launching of outflows and strongly affect their phase structure. Outflows leaving the midplane are denser (ρ ∼ 10^{-26} g cm^{-3}), colder ({∼ } 10^4 K) and slower ({∼ } 30 km s^{-1}) if CRs are considered in addition to thermal SNe. The CR-supported outflows are also smoother, in particular at larger heights (>1 kpc above the midplane) without the direct impact of SN explosions. Approximately, 5 per cent - 25 per cent of the injected CR energy is lost via hadronic cooling. Smaller diffusion coefficients lead to slightly larger hadronic losses but allow for steeper CR pressure gradients, stronger outflows and larger accelerations. Up to a height of z ∼ 1 kpc, there are large volumes in approximate pressure equilibrium between the thermal and the CR component. At larger altitudes, the CR pressure is 10-100 times as large as the thermal counterpart. More than {∼ } 1 kpc away from the midplane, CRs provide the dominant gas acceleration mechanism.