On the Non-Thermal Physics of Magnetic Fields and Cosmic Rays in Galactic Ecosystems

The role of the non-thermal components of galaxies, magnetic fields B) and relativistic charged particles, known as cosmic rays (or CRs), is one of the most uncertain aspects of our understanding of galaxy formation and evolution. While magnetic fields and cosmic rays have long been known to be important components of our own Galaxy, the Milky Way, their part in shaping galactic ecosystems remains elusive. This owes partly to fundamentally indirect observations of physical quantities relevant to B and CRs which are fraught with questionable assumptions to make any physical inference, and partly due to the difficulty in modeling them theoretically. It has only become possible in the past decade to fully model B and CRs dynamically in simulations of galaxy formation within a cosmological context, all while maintaining high hydrodynamic resolution and evolving the relatively well-constrained physics of star formation and stellar feedback to produce realistic bulk- and spatially-resolved galaxy properties without calibration. In this thesis, I use state-of-the-art simulations which explicitly evolve B and CRs in concert with these explicit treatments of star formation and stellar feedback in a cosmological context towards two ends. One is to better understand where our observational assumptions oft used in our indirect constraints may go awry and to develop more physical estimators of B and CRs. In Chapters 2 and 3, I explore this avenue using by generating a host of synthetic observations. The second is to hold the well-constrained (to within an order-of-magnitude) physics fixed, and explore much more widely uncertain physics, namely that of cosmic ray transport, to constrain via emergent observables. In Chapters 4 and 5, I generate synthetic observational predictions across the electromagnetic spectrum for simulations with orders-of-magnitude variation in cosmic ray transport and compare to observations. In Chapter 6, I develop a novel analytic framework to survey the vastly uncertain CR transport parameter space, and explore implications for arbitrarily complex injections of cosmic rays from episodic black hole accretion or star formation, and outline a sub-grid model to incorporate CRs in large volume cosmological simulations which otherwise would suffer from additional computational overhead or artefacts arising from time-independent modeling assumptions. Finally, I conclude summarizing the constraints these various studies provide on galactic B and CRs, and the outlook for future simulations and observational comparisons.