Hunting for Substructure in the Milky Way

Abstract: Studies of the Milky Way including how it formed and evolved through time are the key to understanding how other galaxies behave. The Milky Way is the most convenient galaxy to study, because the close proximity of stars means astronomers can collect precise spectral and dynamical data from individual stars. By combining the chemical and dynamical aspects, it is possible to pinpoint stars which have been accreted from satellite galaxies, in comparison to stars which formed in situ. This constrains the merger history of the Milky Way, which is an important factor in understanding how it has evolved since its formation. In this thesis I have performed a chemodynamical analysis of two sets of data; Fulbright and Nissen & Schuster. I analysed abundance ratios of [Mg/Fe] and [Ni/Fe] relative to [Fe/H] to indicate which stars could be accreted. I then calculated the specific angular momentum, which is the z-component of angular momentum, normalised to a circular orbit: Jz/Jc. I also calculated the specific energy, which is the z-component of the energy, also normalised to a circular orbit: Ez/Ec. From these calculations I analysed the orbits of the stars to predict which stars could be accreted. Subsequently, I combined the two methods into the chemodynamical analysis to identify with confidence how many stars from each sample are accreted. From the Fulbright sample, I successfully identified 18 out of 167 stars which were accreted. From the Nissen & Schuster sample, I successfully identified 29 out of 100 stars which were accreted. These accreted stars are from mergers of low mass dwarf galaxies with the Milky Way, and provide evidence of substructure in the Milky Way.