As galaxies in our Universe form and live out their lives, they exchange matter and energy with their surroundings. Galactic winds are a key component of this exchange of matter and energy. This project aims to study the initiation and propagation of galactic winds with particular focus on the role of particles moving at speeds near the speed of light. Such particles are known as cosmic rays. (In fact, such cosmic rays from our own Milky Way Galaxy interact with each of us every day!) The major outcome of this project will be improved and extended models of galactic winds (1) that build insight into the nature of galactic winds, (2) that can be used by astronomers to connect galaxy and cosmic ray properties with wind properties, and (3) that can be adapted to advanced computer simulations of how galaxies form and change throughout their lifetimes. This project will also strengthen the US science workforce by directly training graduate and undergraduate students in plasma and computational physics and by using the project results to develop teaching materials for university courses. In addition, the investigators will develop a new public outreach program, "The Sun in the Park," that will offer solar viewing and participatory exercises in urban parks in Madison and Milwaukee.
More technically, winds are detected from galaxies of many types. Simulations of galaxy formation and evolution have shown that star formation must be limited by feedback processes and that galactic winds driven by stellar energy and momentum output may be a key part of this feedback. Radio and gamma-ray probes of cosmic rays in a variety of galactic environments have greatly extended our knowledge of cosmic ray properties and those of the underlying galaxies. Although cosmic rays account for a small fraction of the energy output from stars, they have an outsized role in driving galactic winds through their high mobility, long cooling times, and strong coupling to the background through plasma waves. This project will investigate the basis for this coupling in a variety of interstellar environments. This part of the work will address whether cosmic rays couple efficiently to both fully and weakly ionized outflows, how small scale interstellar turbulence affects momentum and energy transfer, and how extreme conditions including high gas densities and weak magnetic fields affect the coupling. The investigators will create a versatile grid of outflow models based on a small number of parameters to aid in interpreting observations of winds and implementing them in galaxy simulations. They will use this work as a basis for improved treatments of cosmic rays in collaborative advanced numerical simulations of galaxy formation and evolution.
