A study of the Hercules dwarf spheroidal galaxy

Detta är en avhandling från Department of Astronomy and Theoretical Physics, Lund University

Sammanfattning: Parts of large spiral galaxies, such as the Milky Way and Andromeda, are believed to have formed from the accretion of several smaller galaxies. Remnants of this chaotic merging era, such as dwarf spheroidal (dSph) galaxies, are visible in the outskirts of large galaxies. dSph galaxies are characterized by their low luminosity, low surface brightness and high fraction of dark matter. Determining the properties, such as mass, luminosity and metallicity, of the dSph galaxies provides key information in our understanding of galaxy formation and evolution. In this thesis, we combine Strömgren photometry with low and high resolution spectroscopy, of the recently discovered Hercules dSph galaxy, to provide information about the evolutionary state of the stars and their radial velocities and metallicities. This enables us to select a clean sample of red-giant branch stars that belong to the Hercules dSph galaxy. Based on the stars identified as members of the Hercules, we find: 1) a galaxy mass that is significantly lower than previous estimates: 2) an abundance trend such that [Ca/Fe] is higher for more metal-poor stars, and lower for more metal-rich stars. This trend suggests an early rapid enrichment through supernovae type II: 3) a tentative evidence for a velocity gradient in our kinematic data. Additionally, we provide a study of the Draco, Sextans and UMaII dSph galaxies using Strömgren photometry. This enables us to establish a new metallicity calibration, based on [Fe/H] determined from high-resolution spectroscopy, from which we derive metallicity distribution functions for the Draco, Sextans and UMaII dSph galaxies.