Dr. Johnston and her co-investigators will perform a comprehensive set of simulations to follow the chemical and dynamical evolution of dwarf galaxies, the most common type of galaxy in our local Universe. The simulations will include a broad range of relevant physics including the buildup of chemical elements heavier than helium, which are produced by nuclear reactions in stars and released to the interstellar gas as the stars end their lives. The team will first identify where a dwarf galaxy forms within a large-scale cosmological simulation, and then re-run that patch of the simulation at much higher resolution with the hydrodynamic code ENZO. This code, largely developed by co-investigator Bryan, provides a higher-fidelity treatment of interstellar gas dynamics and mixing than competing methods. The group will include a new and computationally efficient method to calculate the gas cooling caused by elements with atomic numbers up to Z=30. At its highest resolution, the ENZO code will follow gas dynamics on a scale of 3-4 light years, and can include enough stellar particles to follow all individual stars with at least half the Sun's mass. Dwarf galaxies are normally found close to more luminous galaxies, which can affect their formation and evolution; the team will use their simulations to examine these processes.
A graduate student and a postdoctoral fellow will be trained by their involvement in the research. Dr. Johnston and her co-investigators will produce visualizations from this project, to be shown to the public at the American Museum of Natural History, where Dr. Mac Low is a tenured curator. Dr. Johnston also chairs the Vice Provost's Taskforce for Diversity in Science and Engineering at Columbia, and is active in outreach to schoolgirls.
Galaxies, including our own Milky Way galaxy, come in many sizes, ranging from the giant elliptical galaxies with trillions of stars, to the faintest dwarf systems that may contain just a few thousand. In this project, we used numerical models to investigate how the lowest mass dwarf galaxies form and evolve. This is important both because they represent an extreme form of galaxy formation and so may tell us much about the physics of galaxy formation, but also because they may be relics of the early history of the Universe, and so can probe the conditions that existed at that time. To investigate the formation of these systems we used high-resolution computer simulations using prescriptions for determining when stars formed and the impact of the energy and heavy elements released during their explosions. We were able to reproduce many of the observed properties of these low mass dwarf galaxies, although our simulated dwarfs systematically had too many heavy elements, probably because of the difficulty in modeling the effect of supernovae (in fact, the results of this project have suggested new avenues for solving this problem). We were also able to investigate the impact of various physical effects in the models by running the same simulation with or without each effect. We found that the two most important processes were the energetic input from supernovae, and the processes of reionization, when the combined radiative input from forming stars in the early universe ionized hydrogen. In addition to these computer simulations of dwarf systems, we worked with American Museum of Natural History on a number of their educational and outreach activities. This included participating in the creation of two Planetarium space shows as well as helping to produce content for the Hayden Planetarium’s Big Bang Theater.
