Tree-ring structure of Galactic bar resonance

ABSTRACT 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 disc while growing in volume, thereby sequentially capturing new stars at its surface/separatrix. Since trapped conserve action of libration, which measures relative distance centre, order is preserved: surface dominated captured recently large radius, core occupied early small radius. The thus results a rising mean metallicity from towards centre as Galaxy’s declines radii. This argument, when applied Solar neighbourhood stars, allows novel precision measurement bar’s current pattern speed $\Omega _{\rm p}= 35.5 \pm 0.8 \, {\rm km}\, s}^{-1}\, kpc}^{-1}$, placing corotation radius $R_{\rm CR}= 6.6 0.2 kpc}$. With this speed, precisely fits Hercules stream agreement with kinematics. Beyond corroborating theory, manifests deceleration more than $24{{\ \rm per\ cent}}$ since formation and transfer halo dynamical friction. therefore supports existence standard dark-matter rather alternative gravity.