Cosmic rays in the interstellar medium

Cosmic rays (CRs) are high energy particles with with an energy density comparable to the other energy densities in the interstellar medium (ISM) and very distinct cooling and transport processes. The low cooling efficiency of CRs allows them to travel long distances from their location of acceleration. As a result, they have an impact on star formation at small scales and high densities. Furthermore, the pressure exerted by CRs fills the galaxy, enabling the launching of galactic-scale outflows. These outflows have a significant influence on galactic evolution.

In this review, I will focus on the dynamical impact of CRs, demonstrating how they shape the ISM and contribute to the launching of galactic outflows. There are significant uncertainties in our understanding of CR astrophysics, particularly regarding the speed at which they are transported and the efficiency with which they lose energy. I will highlight the efforts to address these uncertainties by incorporating CRs into hydrodynamical simulations. These simulations aim to accurately model their evolution from non-relativistic to relativistic regimes, while also considering the full magneto-hydrodynamical evolution. By doing so, this improves the accuracy of the CR-driven dynamics and allows us to generate synthetic observables such as gamma rays and synchrotron radiation, which provide valuable insights into the transport and cooling properties of CRs.