Nature or Nurture? Collisionless Evolution of Galactic Disc-Halo Systems

Abstract

In this thesis, we develop and apply a novel algorithm to understand the evolution of stellar discs in ΛCDM cosmological haloes. Three main scientific areas are addressed: the effects of evolving stellar discs on their host haloes, understanding bar formation in cosmological settings, and the formation of vertical disc structure in response to the host halo.

First, we find that the presence of central concentrations of baryons in dark matter haloes enhances adiabatic contraction causes an overall modest reduction in substructure. Additionally, the detailed evolution of stellar discs is important for the inner halo density distribution. However, properties of the halo such as the subhalo mass function are broadly unaffected by the evolution of a realistic stellar disc.

Next, we find that stellar bars invariably form in Milky Way-like galaxies. The strength of these bars is less dependent on properties like the dynamical temperature of the disc in a cosmological setting. Instead, the disc thickness plays a leading role in determining the overall bar strength. Our discs undergo notable buckling events, yielding present-day pseudobulge-disc-halo systems. We show that these are qualitatively similar to the observed Milky Way.

Finally, we show that a wide variety of vertical structure forms when stellar discs are embedded in cosmological haloes. We further show that through a variety of mechanisms, similar vertical structure is excited. By examining twelve simulations of discs in cosmological haloes, we show that the Sgr dSph need not be as massive as 10^{11} solar masses to be consistent with observed vertical structure in the Milky Way. In fact, the recent buckling of the Milky Way’s bar and its present interaction with the LMC are more likely culprits for some of the observed structure in the Milky Way’s thin disc.