A Multifaceted Perspective on Galaxy Evolution

Book excerpt: A key component of astronomy is the study of how galaxies change over time. Once thought to be static "island universes" today, we know that galaxies are dynamic systems reacting to changes in their internal and external environments in myriad ways. From simple observables like their shapes and colors to the complex interplay of their intricate stellar populations, evolving galaxies contain a wealth of information about their past. Yet, these observables alone are not enough to allow us to determine how these galaxies came to be or what they will become. For that, we need to combine theoretical models assembled from fundamental laboratory physics and apply them to galaxies through the use of state- of-the-art computer simulations. This careful combination of observation and computation has allowed our understanding of galaxy evolution to transition from the simple realm of the nebulae into a substantial field of astronomy. In this thesis, we explore three perspectives of galaxy evolution at different levels of detail- through both observations and computer simulations. We begin with a simple observation: the stars in our galaxy appear to be moving in strange ways. If our Galaxy were living an uneventful life, we would expect all of the stars to be moving in nearly circular orbits with small, but appreciable, vertical motions- much like the horse on a carousel. However, recent surveys of nearby stars have found substantial deviations from such a perfect life. With the stars moving vertically in ways that indicate the galaxy has been rung like a bell. Some observers have posited that interactions with very nearby galaxies like the Sagittarius dwarf may be responsible for these unusual stellar motions. In Chapter 2, we use a simulated version of the Milky Way that is undergoing an interaction with a small companion galaxy to explore how such an interaction can affect the motions of stars near the Sun and what effect this may have on the nearly century-old ii problem known as the Oort Limit. Our own galaxy is but one example of an entire population disk-like galaxies with blue colors due to the presence of newly-formed stars. Opposite this population of "blue cloud" galaxies is the "red sequence" which is made up of spheroidal galaxies with red colors due to having nearly no ongoing star formation. In Chapter 3, we take a detailed look at an exceptionally rare, but quite important subpopulation of galaxies that are thought to be transitioning between the blue cloud and the red sequence through the so-called "green valley." Much like transitional fossils in biology, these galaxies have properties intermediate between both the disk-like galaxies of the blue cloud and the massive spheroids of the red sequence. Although few in number, the presence in the universe provides us with a critical view of the fleeting transitions these galaxies are undergoing to help us unlock the mysteries of how massive galaxies in the universe form. Our understanding of galaxy evolution at every scale relies heavily on computer simulations. In Chapter 4, we approach the subject through the lens of a software developer writing a modern N-body solver used to simulate the gravitational dynamics of galaxies. In particular, we explore how utilizing accelerator hardware like graphics processing units (GPUs) can increase both the precision and size of problems that can be solved in galaxy evolution both for today and tomorrow