  Galaxy models have long predicted that galactic bars slow down by losing
angular momentum to their postulated dark haloes. When the bar slows down,
resonance sweeps radially outwards through the galactic disc while growing in
volume, thereby sequentially capturing new stars at its surface/separatrix.
Since trapped stars conserve their action of libration, which measures the
relative distance to the resonance centre, the order of capturing is preserved:
the surface of the resonance is dominated by stars captured recently at large
radius, while the core of the resonance is occupied by stars trapped early at
small radius. The slow-down of the bar thus results in a rising mean
metallicity of trapped stars from the surface towards the centre of the
resonance as the Galaxy's metallicity declines towards large radii. This
argument, when applied to Solar neighbourhood stars, allows a novel precision
measurement of the bar's current pattern speed $\Omega_p = 35.5 \pm 0.8$
km/s/kpc, placing the corotation radius at $R_{CR} = 6.6 \pm 0.2$ kpc. With
this pattern speed, the corotation resonance precisely fits the Hercules stream
in agreement with kinematics. Beyond corroborating the slow bar theory, this
measurement manifests the deceleration of the bar of more than 24% since its
formation and thus the angular momentum transfer to the dark halo by dynamical
friction. The measurement therefore supports the existence of a standard
dark-matter halo rather than alternative models of gravity.
