Book Description

Cosmic rays play a major role in the stability and evolution of galaxies. While they have a fairly small density inside galaxies, their energy density is comparable to the thermal gas and magnetic fields. We now believe that cosmic rays play a major role in galactic and interstellar dynamics, including helping to drive galactic outflows, modify the galaxy's interstellar chemistry, and form large-scale structures. To understand the process of how cosmic rays do this, their microscale interactions with the gas and magnetic fields must be accurate so that their macroscale effects can reproduce observations. Our work in this thesis specifically focuses on how different models of cosmic ray transport change these interactions and their effects. Through the combined approach of analytical solutions and numerical simulations, this thesis aims to better understand how cosmic ray transport affects the stability of the interstellar medium and drive galactic outflows. We begin in Chapter 2 with a linear stability analysis of the Parker instability, a Rayleigh-Taylor like instability with the thermal gas supported against gravity by magnetic fields and cosmic rays. We model three different cosmic ray transport models and find that the model where cosmic rays stream relative to the thermal gas most greatly enhances the instability due to the heating of the thermal gas by cosmic rays scattering off of magnetic fluctuations. We continue with the Parker instability in Chapter 3 where we add radiative cooling to the system and then run numerical simulations with a smooth gravitational potential in 2D and 3D to better understand the nonlinear evolution of the instability in a more realistic environment. When radiative cooling is added, we find it enhances the instability when cosmic rays are locked to the thermal gas while it dampens the instability when cosmic ray streaming is the primary mode of transport. In our MHD simulations, we find that both cosmic ray diffusion and streaming enhance the growth of the instability due to the motion of cosmic rays out of the compressive pockets of gas in the valleys of the magnetic field. While the instability growth seems similar, however, the two transport models result in quite different phase structures of the gas, especially at the top of the Parker loops where streaming cosmic rays heat the gas. We then explore the idea of a cosmic ray Eddington limit in Chapter 4. This theory supposes that cosmic rays, through their own pressure gradient, may be able to overcome hydrostatic equilibrium and launch an outflow if star formation is vigorous enough in that galaxy. For five different galaxies and many different transport models, we find that a cosmic ray Eddington limit does exist. However, the Eddington limit often requires gas densities and/or star formation rates that are far different from typical values for galaxies. Therefore, we conclude that it is unlikely that cosmic rays themselves can reach this Eddington limit and drive a galactic wind. We finally conclude in Chapter 5 with a summary of our results and a short discussion on the future research that could be done based around our conclusions.