| Summary: | <p>This thesis provides a novel dynamical evidence for dark matter by measuring for the first time the deceleration of the Galactic bar. This spin-down of the bar requires the transfer of energy and angular momentum to dark matter by dynamical friction, which not only is in conflict with modified gravity theories, but also imposes constraints on the kinematics and nature of dark matter.</p>
<p>The deceleration of the Galactic bar is quantified using the astrometric and photometric data of the Solar neighbourhood stars recently obtained by the Gaia space telescope. When the bar spins down, the resonant phase-space drifts radially outward through the stellar disk while growing in volume, thereby capturing new stars along its path. Since these trapped stars conserve the action of libration, which quantifies the amplitude of motion around the resonance, the resonant phase-space grows just like tree-rings, adding new layers of trapped stars at its surface. Together with the negative metallicity gradient in the disk, this implies a prominent increase in stellar metallicity towards the resonance centre. We detected this tell-tale signal in the bar's corotation resonance, thus quantitatively proving the bar's slowdown. During the course of this research, we also resolved the long-standing issue on the origin of the Hercules stellar stream and provided a precise estimate on the bar's current pattern speed.</p>
<p>Furthermore, in order to constrain the phase-space distribution of dark matter from the measured slowing rate of the bar, we developed a theory of dynamical friction in the non-linear regime in the slow limit of bar evolution. Using the resonant angle-action coordinates, we successfully modelled the long-term oscillation and saturation of dynamical friction as a result of the non-linear phase mixing in the trapped phase-space.</p>
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