Cosmic Ray Feedback on the ISM: The Effects of Variable Cosmic Ray Propagation

Cosmic rays created in the shock waves of supernovae are confined to the galactic disk for an average of ~ 3 Myr, where their energy density is similar to the magnetic field and turbulent motion in the ISM, making cosmic rays a dynamically important constituent. Once cosmic rays propagate out of the disk, there is a transition in their propagation from a diffusive random walk to advection with the gas or streaming at the Alfven speed outside of the disk. Additionally, the propagation method determines how the cosmic rays affect the thermal plasma. The feedback of cosmic rays on the thermal plasma can impact the launching of galactic winds and overall structure of the interstellar medium and circumgalactic medium. How does this spatial switch in cosmic ray propagation, and resultant change in cosmic ray feedback, change the structure of a galactic disk? We use cosmic ray magnetohydrodynamic simulations (Athena++) to study how this variable cosmic ray propagation affects galactic disk structure on long times scales (t ≥ 200 Myr). The simulations use a stratified box model, initially in hydrostatic equilibrium. We then inject thermal energy and cosmic ray energy in localized spots throughout the midplane of the disk, approximating the effects of supernovae on the thermal gas. We also include optically thin radiative cooling, producing a multiphase gas. We will present preliminary results along with our implementation of variable cosmic ray propagation, which depends on local plasma properties.