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

Abstract

We investigate the impact of cosmic rays (CRs) on galactic outflows from a multi-phase interstellar medium with solar neighbourhood conditions. The three-dimensional magneto-hydrodynamical simulations include CRs as a relativistic fluid in the advection-diffusion approximation. The thermal and chemical state of the ISM is computed with a non-equilibrium chemical network. We find that CRs (injected with 10 \% of the supernova energy) efficiently support the launching of outflows and strongly affect their phase structure. Outflows leaving the midplane are denser ($\rho \sim 10^{-26}\,\mathrm{g\,cm}^{-3}$), colder ($\sim 10^4\,\mathrm{K}$), and slower ($\sim 30\,\mathrm{km\,s}^{-1}$) if CRs are considered in addition to thermal SNe. The CR supported outflows are also smoother, in particular at larger heights ($> 1\,\mathrm{kpc}$ above the midplane) without the direct impact of SN explosions. Approximately $5\% - 25\%$ 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 $z \sim1\,\mathrm{kpc}$ there are large volumes in approximate pressure equilibrium between thermal and CR component. At larger altitudes the CR pressure is $10-100$ times as large as the thermal counterpart. More than $\sim 1\,\mathrm{kpc}$ away from the midplane, CRs provide the dominant gas acceleration mechanism.