The Slowing Bar

In the inner Milky Way stars trek together in a bar-shaped, rotating overdensity: the central bar, which dominates the inner disc out half-way to the Sun. The rotation velocity/pattern speed of this bar has been intensely debated. The best insight into the bar's motion are gained from its resonant effects on the Milky Way disc:

Similar to Jupiter holding in resonant motion Solar System asteroids like Greeks and Trojans, some stars near the Sun are locked in resonant motion, e.g. in corotation resonance circling the Lagrange points on the bar's minor axis. When the bar slows, these resonances move outwards through the Galaxy dragging bound stars along. Along their path they can pick up new stars. The corotation resonance is enhanced by this process, explaining the high contrast of the observed Hercules stream. My group has used this to measure for the first time the slowing of the bar's rotation. We have also resolved long-term issues with bar pattern speed measurements. This also promises new insighs into the Milky Way's history. Most importantly, the bar's slowing can only be explained with angular momentum loss to the Milky Way's dark matter halo, thereby proving its inertial mass.