The Microphysics of Cosmic-ray Feedback

Abstract: Cosmic-rays (CRs) are an important constituent of the interstellar medium and likely provide crucial dynamical feedback to their host galaxies and the corresponding circumgalactic medium. Besides providing pressure support and being naturally buoyant that tend to escape, the key microphysics in CR feedback lies in the gyro-resonant CRs streaming instabilities (CRSI). However, existing global simulations of CR feedback at galactic scales generally rely on highly simplified approximations of the CR physics, and may yield dramatically different results depending on the prescription employed. I will describe the first numerical studies of the CRSI from first principles at microscopic level. CRs streaming through background magnetic fields makes Alfv\’en waves unstable when the bulk streaming speed exceeds the Alfven velocity. The subsequent wave growth scatters CRs via quasi-linear diffusion, and eventually isotropizes the CRs in the wave frame. This process leads to momentum exchange between CRs and background fluid, and is the key behind CR-driven outflows. The long-term goal of this study is to provide reliable subgrid models for future studies of CR feedback at galactic scales.