The GALAH and GAIA surveys: the response of the Galactic disk to non-equilibrium features

Abstract

The dynamical history of the Milky Way is complicated. We see a lot of substructure (that is likely accreted), both in spatial density, and in kinematics. The complexity varies, from simple streams to phase-mixed structures that can only be separated by invariants such as stellar chemistry. To complicate things further, the Galaxy has non-axisymmetric structures such as the bar and multiple spiral arms. Due to their large scale-lengths, the kinematic signatures of such features often overlap, making it difficult to relate to one culprit or another. With the aid of high precision spectroscopy and astrometry, combined with dynamical modelling, we can make an attempt at understanding the various underlying causes of non-axisymmetric motions in the Galaxy. In this thesis, we adopt this multi-pronged approach that combines predictive models with high precision data. We explore the signatures of non-axisymmetry on both the local and global phase-space in the Galaxy. In the first part, we use kinematical modelling to quantify bulk motion in the line-of-sight velocity field of the disc, and place limits on the derived amplitudes accounting for systematics/limitations in the data. In the second part, we reveal new global phase-space structure in the Galaxy, and study it in the context of the perturbative effects of both internal (bar/spiral structure) and external (satellite galaxies) sources. Our findings suggest that the nature of spiral arms in our Galaxy might be transient.