The bar resonances and low angular momentum moving groups in the Galaxy revealed by stellar ages.

We use the second Gaia data release in combination with the catalog of Sanders & Das (2018) to dissect the Milky Way disc in phase-space and relative ages. We confirm and report the existence of multiple velocity moving groups at low azimuthal velocities and angular momenta, below Arcturus, regularly separated by $\sim~18-20\,\rm{km s^{-1}}$ in azimuthal velocity. Such features were predicted to exist more than ten years ago from the reaction of the Milky Way to a perturbation in the disc undergoing phase-mixing. These structures appear slightly younger than their phase-space surroundings, arguing against an extra-galactic origin. We also identify in relative age many of the classical ridges in the plane of azimuthal velocity vs. Galactocentric radius, traditionally associated with resonance features. These ridges are also younger than their phase-space surroundings in accordance with predictions from recent state-of-the-art cosmological hydrodynamical simulations of Milky Way-like galaxies. We study the response of dynamically young and old stellar disc populations to resonances from an analytic model of a large bar, which, remarkably, qualitatively reproduces the trends seen in the data. Our results re-inforce the idea that the Galactic disc is currently being shaped by both internal and external perturbations, and that, while absolute isochrone ages have to be taken with great care, exploring the dynamical structure of the disc in stellar ages, especially with future asteroseismic data, will provide much stronger constraints than metallicity/abundance trends alone.