Letter to the Editor

Bar resonances and low angular momentum moving groups in the Galaxy revealed by their stellar ages

¹ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, Chiba 277-8583, Japan
e-mail: chervin.laporte@ipmu.jp
² Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Bd de l’Observatoire, 06304 Nice, France
³ Department of Physics & Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada
⁴ Université de Strasbourg, CNRS UMR 7550, Observatoire Astronomique de Strasbourg, 11 Rue de l’Université, 67000 Strasbourg, France
⁵ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany

Received: 24 June 2020
Accepted: 27 August 2020

Abstract

We use the second Gaia data release 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 ∼18−20 km s⁻¹ in azimuthal velocity. Such features were predicted to exist more than ten years ago, based on 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 and reach up to high (solar) metallicities, which argues against an extra-galactic origin. We also identify, in terms of relative age, many of the classical ridges in the plane of azimuthal velocity versus Galactocentric radius, which are 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, reproduces qualitatively the trends seen in the data for the classical ridges close to circularity. Our results reinforce the idea that the Galactic disc is being shaped by both internal and external perturbations, along with the fact 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 and abundance trends alone.