2014 Theses Doctoral

Dwarf Galaxies in a Cosmological Context: A Testbench for Galaxy Modeling

Simpson, Christine Mary

Presented here are simulated models for the evolution of a 10^9 M. dark matter halo in a cosmological setting with an adaptive mesh refinement code as an analog to local low-luminosity dwarf irregular and dwarf spheroidal galaxies. The primary goals of this study are to investigate the roles of reionization and supernova feedback in determining the star-formation histories of low-mass dwarf galaxies and to explore the effect of differing numerical implementations of supernova feedback on galactic enrichment and winds. Our models include a wide range of physical effects, including metal cooling, molecular hydrogen formation and cooling, photoionization and photodissociation from a metagalactic (but not local) background, a simple prescription for self-shielding, star formation and two different models for supernova-driven energetic feedback. To better understand the impact of each physical effect, we carry out simulations excluding each major effect in turn. We find that reionization is primarily responsible for expelling most of the gas in our simulations, but that supernova feedback is required to disperse the dense, cold gas in the core of the halo. Moreover, we show that the timing of reionization can produce an order-of-magnitude difference in the final stellar mass of the system. While the stellar masses produced in our models with purely thermal supernova feedback are consistent with observed low-luminosity dwarfs, the resulting median stellar metallicity is considerably larger than observed systems. We investigate the efficacy of purely thermal energetic feedback, and suggest that it may still suffer from excessive radiative losses, despite reaching stellar particle masses of about 100 Msun and a comoving spatial resolution of 11 pc. We investigate a second model for supernova feedback that includes kinetic as well as thermal energy in the proportions predicted by Sedov-Taylor models on the resolution scales of our galaxy simulations. We extensively test the effect of this model in media of different densities and at different resolutions and we conclude that the inclusion of kinetic energy is most important in dense gas simulated at low resolution. The effect of this new model on our simulated dwarf galaxy is significant, as it produces stronger galactic winds that suppress and regulate star formation and more efficiently eject metals from star forming gas. The resulting system at z = 0 has an order of magnitude lower luminosity and an average stellar metallicity consistent with observed dwarfs. The distribution of stellar metallicity is too narrowly peaked, however, indicating the need for further refinement of our model and perhaps the inclusion other sources of stellar feedback such as Type Ia supernovae or stellar winds. We conclude that the observed chemical abundance patterns in local dwarf galaxies provide a unique testbench for refining models of stellar feedback in galaxy simulations at high resolution.