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Simulations of Galaxy Formations and Evolution – Brian O’Shea

Theoretical astrophysics professor Brian O’Shea, from the Department of Physics and Astronomy and Lyman Briggs College at Michigan State University, is interested in the formation of cosmological structure in the universe – in particular, how galaxies form, evolve, and interact with their surroundings. He studies these phenomena using sophisticated, physics-rich simulations on MSU’s High Performance Computing Center, and also on NSF, DOE and NASA supercomputing resources. Currently, his group’s research focuses around three main themes.

1. The formation of the earliest galaxies. The first generations of stars and galaxies are important because they set the stage for all later galaxy formation: these were the first places where metals were created and distributed throughout the Universe, are potential seed sites for the supermassive black holes that are observed at the center of every modern-day massive galaxy, and are the objects that started the process of “reionization,” which converted the vast majority of gas in the Universe from a cold, neutral state to a hot, ionized state. This last process started when the first star in the Universe was formed, and ended approximately one billion years after the Big Bang. Observing galaxies in the pre-reionization epoch is a major observational challenge, and the next generation of ground and space-based telescopes are being constructed with this goal in mind. Making theoretical models of the earliest galaxies is crucial to interpreting these observations, and will yield important insights into galaxy formation and evolution that will be useful for understanding later, larger galaxies such as our own Milky Way. Dr. O’Shea’s research on this subject focuses on the transition from metal-free to metal-enriched star formation (where stars are expected to have fundamental changes in their properties), the formation of galaxies from these resulting stars, and then predictions for the next generation of ground- and space-based telescopes, such as the James Webb Space Telescope.

2. Galaxies in extreme environments. Galaxy clusters, composed of tens or hundreds of galaxies orbiting within a single common dark matter halo, are the largest gravitationally-bound objects in the Universe, often weighing more than 100 times the mass of the Milky Way. As the largest bound objects, galaxy clusters are useful probes of cosmology and are very interesting astrophysical laboratories in that they are essentially “closed box” systems. 90% of the baryonic matter in galaxy clusters is outside of the galaxies themselves, residing in a hot, diffuse plasma called the “intracluster medium,” or ICM, which is extremely bright at X-ray wavelengths but invisible to the naked eye. This plasma is threaded with magnetic fields and relativistic protons and electrons, which are crucial to controlling its behavior. Understanding the ICM and its interactions with the galaxies contained within it is crucial to gaining a complete understanding of galaxy clusters as a whole, the life cycles of these objects, and to assessing their utility as cosmological probes. Dr. O’Shea’s research on this subject focuses on the non-thermal evolution of the intracluster medium, including the effects of magnetic fields and cosmic rays; the effect of feedback from stellar populations and active galactic nuclei, particularly on radio, gamma-ray and x-ray observables; the effects of non-thermal processes in the ICM on galaxy clusters as cosmological probes; and alternative methods of simulating galaxies within clusters.

3. Galactic chemical evolution. Starts that form in the Milky Way and in other galaxies come in a wide range of sizes – from a fraction of the size of the Sun to more than 100 times its mass. When the more massive stars reach the end of their lives, they die in energetic explosions (known as supernovae) and produce all of the elements on the periodic table heavier than Lithium. Somewhat less massive stars (that are still larger than our own Sun) also can produce large amounts of these elements as well. One can use our understanding of stellar and nuclear processes, and observations of metal-poor stars in our galactic halo (by surveys such as the Sloan Digitcal Sky Survey’s SEGUE project), to probe the chemical and dynamical evolution of our own Milky Way. Doing this provides insight into both how the Milky Way formed and also the properties of the older, smaller galaxies that merged together to make the galaxy we live in today. This work is an essential complement to studies of the most distant galaxies in the universe, since it allows us to probe very different aspects of the structure formation process. Dr. O’Shea’s research on this subject focuses on high-fidelity simulations of chemical evolution, including evolving stellar populations; improved models for star formation and feedback in cosmological simulations; using metal-poor galactic halo stars as probes of the Milky Way chemical and dynamical evolution; and the development of statistical tools to compare models with large-scale observational datasets.

A single simulation could cost five million CPU-hours, and produce hundreds of terabytes of data. These calculations require the resources of HPCC and other supercomputing centers. This work has paid off in the form of many peer-reviewed journal articles.

– Michigan State University Institute for Cyber-Enabled Research

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