Effective cosmic-ray diffusion in multiphase galactic environments

Cosmic-ray (CR) feedback is widely recognized as a key regulator of galaxy formation. After being accelerated at supernova remnant shocks, CRs propagate through the interstellar medium (ISM), establishing smooth large-scale distributions and driving galactic outflows. The efficiency of this feedback is controlled by the effective transport speed of the CR population, which in turn depends on the competition between CR-driven plasma instabilities and wave-damping processes that vary strongly with ISM phase. In cold, dense gas, ion-neutral damping dominates, whereas in warm, diffuse environments, weaker non-linear Landau damping prevails, leading to enhanced CR scattering and slower transport. To investigate these effects, we employed the moving-mesh code AREPO and modelled CR transport using a two-moment description within the multiphase ISM framework C RISP , which self-consistently computes CR diffusion coefficients and transport velocities from coarse-grained plasma physics. The intrinsic CR diffusion coefficient depends inversely on the scattering rate of CRs and Alfvén waves, covering 15 orders of magnitude. In contrast, we show that the effective CR diffusion coefficient, which quantifies the propagation speed of CRs through the ISM, converges towards the canonical range of 10 28 − 10 29 cm 2 s −1 . Simulations with non-linear Landau damping alone yield transport rates that are slower by up to an order of magnitude than those including both Landau and ion-neutral damping. Overall, the CR transport speeds increase systematically with gas density, for which we provide a density-dependent fit of the effective CR diffusion coefficient. We demonstrate that despite strong ion-neutral damping in the cold and warm phases of the galactic disk, CRs rapidly smooth out their gradient and are transported at speeds of only a few times the local Alfvén speed as they traverse alternating ISM phases on their way out of the galaxy. This approach enables us to investigate how self-consistent CR transport affects CR induced non-thermal observables in the future.