Cosmic ray transport and the galaxy gas cycle

Author / Creator

Bustard, Chad, author

Available as

Online

Physical

Summary

How galaxies form and evolve is dictated, in large part, by the galaxy gas cycle: a balance of inflows from the circumgalactic medium (CGM) and large-scale outflows (or winds) driven by active gala...

How galaxies form and evolve is dictated, in large part, by the galaxy gas cycle: a balance of inflows from the circumgalactic medium (CGM) and large-scale outflows (or winds) driven by active galactic nuclei or collections of supernovae from the galaxy's interstellar medium (ISM). Understanding this cycle is a long-standing challenge in cosmology, and a complete description must connect a vast range of scales, from large-scale accretion hundreds of kiloparsecs in scale down to sub-parsec star-forming scales and beyond. Cosmic rays, highly-energetic charged particles that traverse the Universe along magnetic fields, are an important component of this multi-scale problem. Despite being far lower density than normal gas, cosmic rays have enough energy to shape the structure of the ISM and the gas flows that emanate from it, and they do so through a microscopic (scales < 1 AU) coupling to magnetic waves that is well-described by plasma kinetic theory. In this thesis, we leverage novel numerical techniques and our modern, plasma-physics-based understanding of micro-scale cosmic ray transport to study its macroscopic effects on galaxy evolution. We start in Chapter 2 with a simplified model of thermally driven galactic winds, exploring the parameter space of wind mass- and energy-loading factors and comparing our results to galaxy X-ray observation trends. We find that moderately mass-loaded outflows rapidly cool in-situ and develop cold gas reservoirs in the galaxy halo. This complements expulsion of cold ISM gas directly from the disk, helping to account for the puzzling abundance of low-ionization species observed in outflows. We then use this same model to estimate cosmic ray acceleration at galactic wind termination shocks in Chapter 3. We find that, while starburst galaxies may lack the power to accelerate ultra-high energy cosmic rays, they may act as a natural source for cosmic rays between 10^(15) and 10^(18) eV (the unexplained "shin" of the cosmic ray spectrum) and would project a significant cosmic ray flux into the CGM. We then utilize the FLASH MHD code, with an additional cosmic ray transport module, to study both thermally (Chapter 4) and cosmic ray driven (Chapter 5) outflows from the Large Magellanic Cloud (LMC), a dwarf satellite galaxy of the Milky Way. We show that an edge-on ram pressure can expel even weak fountain flows from the LMC's gravitational potential well, but the formation of a near-side halo and trailing outflow plume is inhibited when the LMC's infall is face-on. In our simulations, cosmic rays are essential for driving a realistic outflow, they become energetically dominant (compared to thermal gas) in the extended galaxy halo, and their flux into the CGM is high. We provide preliminary mock observations by decomposing the system into neutral and ionized hydrogen. We also comment on the overproduction of gamma-rays through hadronic collisions, both in our LMC simulations and other state-of-the-art galaxy evolution simulations, compared to observations. These combined studies raise new questions and motivate an abundance of future studies regarding the interpretation of observations, the flow of gas and cosmic rays through the disk-halo interface, and the nature of cosmic ray transport (see the Conclusions section).

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