The Dynamics and History of the Milky Way

Abstract

This thesis uses a combination of observations, simulations, and dynamical analysis to study the dark matter (DM) halo and accretion history of the Milky Way (MW).

Chapter 2 presents and applies a method to infer the mass of the MW by comparing the dynamics of observed satellites to those in the EAGLE cosmological hydrodynamics simulations. The method is robustly calibrated on mock EAGLE systems and then validated on the independent AURIGA high-resolution simulation suite of 30 MW-like galaxies. The method is shown to recover a galaxy's true mass and associated uncertainties accurately. Using ten classical satellites with 6D phase-space measurements, including updated proper motions from the Gaia satellite, the method is applied to the MW. The mass of the MW is estimated to 15 percent error. This is then combined with recent mass estimates in the inner regions of the Galaxy to infer an NFW halo concentration.

Chapter 3 models the effect of baryonic contraction in DM haloes by first studying the orbital phase-space of DM haloes in the AURIGA simulation suite of MW analogues. The haloes are characterised by their spherical action distribution of the DM particles, allowing a comparison between the dynamical descriptions of DM-only and hydrodynamical simulations of the same haloes. By applying an iterative algorithm, the AURIGA DM haloes are adiabatically contracted to a given baryon density profile and halo mass. Using this algorithm, the AURIGA haloes are contracted to the baryon profile of the MW, inferring the total mass profile and the dynamics of the MW's contracted DM halo. These models allow the derivation of updated values for the key astrophysical inputs to DM direct detection experiments: the DM density and velocity distribution in the Solar neighbourhood.

Chapter 4 introduces a multi-component chemo-dynamical model for decomposing the Galactic population of Globular Clusters (GCs) into bulge, disc, and stellar halo components. The halo GCs are further split into the major Galactic accretion events, GES, Kraken, Sequoia, Sag, and Helmi streams. The modelling approach is extensively tested using mock GCs built using the AURIGA suite of hydrodynamical simulations. The method is applied to the Galactic GCs data to infer, in a statistically robust and easily quantifiable way, the GCs associated to each MW accretion event. The MW sample contains 170 GCs, including previously uncategorized clusters from Gaia EDR3 observations. The number of GCs if each accretion group is then used to infer properties, such as halo and stellar masses, of these defunct satellites of the MW.