Formation and dynamics of the Galactic disc and halo
The treasure trove of recent data from Gaia and spectroscopic surveys has revealed an abundance of features in the Milky Way that must be interpreted dynamically. The disc, the bar, the halo and the satellites are all components of the Galaxy's dynamical system which are able to interact to create, perturb and destroy substructure. By using tailored and cosmological simulations, this substructure can be modelled and interpreted to learn about the formation and past evolution of the Milky Way. A detailed understanding of our own Galaxy's assembly also provides a test for the cosmological models implemented in hydrodynamic simulations of Milky Way analogues. In Chapter 1 I give an overview of the principles of Galactic dynamics which form the foundation of this thesis. I proceed to outline past research into Galactic archaeology in the Milky Way, including studies into its substructure, evolution and accretion history. Chapter 2 is a study of the dynamical effects of the Sagittarius Dwarf Galaxy (Sgr) on stellar streams produced by tidally disrupting globular clusters, with a focus on the GD-1 stream. This reveals a plethora of perturbations that may result from interactions with a massive Sgr, including asymmetry and folding of the tails. In Chapter 3 I run simulations of a stellar halo-like population of particles in the presence of a rotating bar. I show that the bar is capable of trapping stars in its resonances from highly eccentric and inclined orbits. These form overdensities in phase space which closely resemble similar features seen in data from Gaia. Chapter 4 continues to investigate the trapping of halo orbits in resonances with the bar. I develop an analytic toy model of resonant orbits to predict the appearances of action space (J_φ, J_r) and radial phase space (r, v_r). Both this model and test particle simulations successfully predict the appearance of Gaia data in these spaces, in particular the asymmetry in Galactocentric radial velocity v_r. Chapter 5 is a study of globular clusters trapped in resonances with the bar. I select 10 whose orbits are likely to be significantly altered by a bar with a slowing pattern speed, most of which are trapped by the corotation resonance. I show that a slowing bar is capable of transporting these clusters to higher energies and radii, indicating that it may have reshaped the Galaxy's globular cluster system. In Chapter 6 I use the ARTEMIS suite of cosmological simulations to investigate the angular momenta of dark matter haloes of Milky Way analogues. I find a correlation between the spin of a halo and its accretion history. Haloes with a highly radial accreted stellar component (like Gaia Sausage-Enceladus in the MW) tend to have lower spin than those with more isotropic accreted stellar velocity distributions. Chapter 7 uses the ARTEMIS simulations to study the formation (spin-up) of the Milky Way's disc. Most simulated galaxies spin up at higher metallicity than the Milky Way, suggesting our galaxy formed its disc unusually early. Galaxies with an analogue of Gaia Sausage-Enceladus spin up earlier than those without on average. The early spin-up galaxies tend to have both greater virial masses at early times and smaller present-day fractions of accreted stars.
Milky Way, Galactic Dynamics, Galactic Archaeology, Gaia
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