Galactic Disc Dynamics and Structure : Analysis of the Phase Spiral and Vertical Scale-heights

The Milky Way provides the most detailed laboratory we have access to for studying the processes of galaxy formation and evolution. Modern large-scale surveys, most notably the Gaia mission, have revolutionised this field by revealing the extent to which the Galaxy is not a steady system, but dynamically active and rich in non-equilibrium features that preserve traces of past interactions. Understanding these structures, and how they relate to the Galaxy’s broader history, is central to thefield of Galactic archaeology — the reconstruction of the Milky Way’s past from the evidence preserved in its structures and stars.This thesis investigates the structure and dynamical state of the Milky Way using astrometric data from Gaia and spectroscopic data from the APOGEE survey. This thesis aims to characterise the Gaia phase spiral in the Galactic disc and assess how different methods of stellar classification affect our understanding of the Galaxy’s large-scale structure.The first paper presented in this thesis analyses the Gaia phase spiral, a characteristic pattern in the vertical positions and motions of stars, and quantifies its strength and rotation across the disc using a new model. The results show that the spiral is a rotating feature whose amplitude increases with angular momentum. The second paper investigates the two-armed phase spiral observed near the Solar neighbourhood, measures its geometry and rotation and explores whether it represents a single two-armed perturbation or two overlapping ones.The third paper compares several methods for separating the thin and thick discs based on chemistry, kinematics, dynamics, and stellar ages. The comparison shows that the detailed properties of the components depend on the chosen method, underscoring the need for clarity in the selection of definitions.