Chad Bustard
Postdoctoral Fellow @ Kavli Institute for Theoretical Physics (KITP)
Cosmic Rays
Cosmic rays, though few in number compared to normal gas, are an energetically significant component of galaxies, and their effects are felt in a number of astrophysical environments. How these non-thermal particles attain such great energies, how they couple to magnetic fields, and how they exchange energy with thermal gas is necessarily described by plasma physics.
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What accelerates cosmic rays depends on the energy range of interest: at low energies, there is strong theoretical and observational evidence that they are accelerated by supernova remnant shock fronts within our Galaxy; at high energies, cosmic rays are likely accelerated by more energetic extragalactic sources. At intermediate energies, which some refer to as the "shin" of the cosmic ray spectrum between the "knee" and the "ankle," the dominant source class is a mystery. Shocks formed by galactic winds running into the intergalactic medium may be a viable source (Bustard+ 2017, Merten+ 2018).
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Since galactic, ~GeV energy cosmic rays comprise the peak of the cosmic ray energy spectrum and therefore have the most influence on galaxy evolution, much of my research focuses on their transport and influence in multiphase galactic and circumgalactic environments. To get a complete picture, I believe it's important to study cosmic rays in not only galaxy-scale simulations (Bustard+ 2020) but also idealized, high-resolution simulations (Heintz+ 2020, Bustard+ 2021)
Cosmic rays impinging upon a cold, dense cloud hit a bottleneck and develop a pressure gradient. Magnetic field lines, easily warped around the cloud by pressure perturbations, are shown as streamlines. How streaming cosmic rays navigate and accelerate cold clumps is a fundamental problem we want to answer with high-resolution, idealized simulations (Bustard and Zweibel 2021)
Edge-on view of a simulated cosmic ray driven wind from the Large Magellanic Cloud (Bustard+ 2020)
Cosmic ray luminosity for different wind/shock models. For high velocity winds, this results in luminosities that can be a significant fraction of the Milky Way luminosity, suggesting that termination shocks may substantially add to a galaxy's cosmic ray energy output. (Bustard+ 2017)
Cosmic rays impinging upon a cold, dense cloud hit a bottleneck and develop a pressure gradient. Magnetic field lines, easily warped around the cloud by pressure perturbations, are shown as streamlines. How streaming cosmic rays navigate and accelerate cold clumps is a fundamental problem we want to answer with high-resolution, idealized simulations (Bustard and Zweibel 2021)
Relevant Publications
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