The Impact of Cosmic Ray and Ionizing Radiation on Galactic Structure and Associated Observational Diagnostics

Abstract

Observations of galaxies have uncovered a complex, multiphase galactic environment structured by the effects of feedback mechanisms within and beyond the galaxy. Still, the exact properties of the galactic and circumgalactic medium remain uncertain. Plasma properties derived from diagnostic ions depend on the ionization mechanism assumed, for example collisional or photoionization. In other words, proper modeling of the radiation field is important. By analyzing cosmological galaxy simulations, I find that that the stellar contribution to the radiation field generally dominates up to roughly 0.1 - 0.2 Rvir, so analyses within that radius should consider stellar sources in ionization assumptions. At the same time, cosmic rays have emerged as a potentially significant feedback mechanism, acting as a non-thermal pressure source, as well as imparting energy and momentum into gas. It is still unclear as to which model of cosmic ray transport is the most realistic. I conduct three dimensional simulations of a section of a Milky Way-like galactic disk. I study a model of cosmic ray transport that includes the effects of the turbulent and magnetized medium, and show that the galactic wind properties are sensitive to the strength of turbulence. Furthermore, I conduct similar galaxy disk section simulations, focusing on both radiative and cosmic ray feedback. I describe methods for producing synthetic spectral line diagnostics from highly-ionized species in simulations and find that models of fast cosmic ray diffusivity without cosmic ray streaming heating best agree with observed diagnostic values. This work illustrates how ultraviolet diagnostics could be useful tools to help differentiate between radiative and cosmic ray feedback models.