Cosmic Rays and the Circumgalactic medium
Figure 1: A (simplified) schematic of cosmic rays in the CGM. Galactic supernovae inject roughly 10% of their energy as cosmic-ray energy. Those cosmic rays move away from their injection site along tangled magnetic fields, creating a cosmic-ray pressure gradient that helps maintain hydrostatic equilibrium. This cosmic-ray pressure may be a significant, or even the dominant pressure source in the CGM around L* galaxies, however, the quantitative details (e.g., the exact cosmic-ray pressure or its radial extent) are sensitive to models of cosmic-ray transport, which remain unconstrained.
Cosmic rays fundamentally alter CGM structure
Galaxies evolve embedded in a vast gaseous halo that dwarfs the mass and spatial extent of its stars. In order to understand galaxy evolution, we must first understand the complex interplay between galaxies and their circumgalactic medium (CGM).
Much of my recent work has pioneered our understanding of how cosmic rays fundamentally alter the structure of the CGM, especially around low-redshift L* galaxies. Cosmic rays drive cool, mass-loaded outflows that enhance the CGM column densities of many metal ions (Butsky and Quinn 2018). Once in the CGM, cosmic-ray pressure support alters the morphology of cool gas, leading to large, low-density clouds that are out of thermal pressure equilibrium with the hot gas (Butsky et al. 2020). This effect leads to detectable differences in the kinematic signatures of multiphase CGM gas (Butsky et al. 2022).
While there is still much work to be done to robustly quantify the impact of cosmic rays on the CGM ( check out my work on constraining cosmic-ray transport!), it has become clear that the complex coupling between cosmic rays and (circum)galactic gas can significantly affect galaxy evolution.