The impact of astrophysical dust grains on the confinement of cosmic rays

Abstract: We argue that charged dust grains could significantly impact the confinement and transport of galactic cosmic rays. For sub-GeV to ~1000GeV cosmic rays, small-scale parallel Alfv\'en waves, which isotropize cosmic rays through gyro-resonant interactions, are also gyro-resonant with charged grains. If the dust is nearly stationary, as in the bulk of the interstellar medium, Alfv\'en waves are damped by dust. This will reduce the amplitude of Alfv\'en waves produced by the cosmic rays through the streaming instability, thus enhancing cosmic-ray transport. In well-ionized regions, the dust damping rate is larger by a factor of ~10 than other mechanisms that damp parallel Alfv\'en waves at the scales relevant for ~GeV cosmic rays, suggesting that dust could play a key role in regulating cosmic-ray transport. In astrophysical situations in which the dust moves through the gas with super-Alfv\'enic velocities, Alfv\'en waves are rendered unstable, which could directly scatter cosmic rays. This interaction has the potential to create a strong feedback mechanism where dust, driven through the gas by radiation pressure, then strongly enhances the confinement of cosmic rays, increasing their capacity to drive outflows. This mechanism may act in the circumgalactic medium around star-forming galaxies and active galactic nuclei.