Probing dark matter with resonant dynamics of the Galactic bar

I will present a new evidence for dark matter from the measurement of the deceleration of the Galactic bar. Galactic bars are linear structures that form and rotate at the center of galactic disks. In the presence of dark matter haloes, these bars are predicted to gradually spin down by gravitationally transferring their energy and angular momentum to dark matter, a process known as dynamical friction. In contrast, MOND theories predict no bar slowdown since disk stars alone cannot provide enough friction on the bar. I will demonstrate that the slowdown of the Galactic bar can be inferred from the stars trapped in resonance with the bar: just like annual tree-rings, the resonant island in phase space evolves inside-out, capturing new stars by expanding its surface while conserving the internal distribution. Using the recent data from Gaia, I will show that the bar's corotation resonance bears this tree-ring structure, allowing us to infer the bar's slowdown history. In the latter part of my talk, I will also present a new approach to model dynamical friction in the realistic nonlinear regime.