The evolution of the Milky Way disk, which contains most of the stars in the Galaxy, is affected by several phenomena. For example, the bar and the spiral arms of the Milky Way induce radial migration of stars1 and can trap or scatter stars close to orbital resonances2. External perturbations from satellite galaxies can also have a role, causing dynamical heating of the Galaxy3, ring-like structures in the disk4 and correlations between different components of the stellar velocity5. These perturbations can also cause ‘phase wrapping’ signatures in the disk6,7,8,9, such as arched velocity structures in the motions of stars in the Galactic plane. Some manifestations of these dynamical processes have already been detected, including kinematic substructure in samples of nearby stars10,11,12, density asymmetries and velocities across the Galactic disk that differ from the axisymmetric and equilibrium expectations13, especially in the vertical direction11,14,15,16, and signatures of incomplete phase mixing in the disk7,12,17,18. Here we report an analysis of the motions of six million stars in the Milky Way disk. We show that the phase-space distribution contains different substructures with various morphologies, such as snail shells and ridges, when spatial and velocity coordinates are combined. We infer that the disk must have been perturbed between 300 million and 900 million years ago, consistent with estimates of the previous pericentric passage of the Sagittarius dwarf galaxy. Our findings show that the Galactic disk is dynamically young and that modelling it as time-independent and axisymmetric is incorrect.
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The datasets used and analysed for this study are derived from data available in the public Gaia archive (https://gea.esac.esa.int/archive). The Bayesian distances for the Gaia sources with radial velocity37are available at http://www.astro.lu.se/~paul/GaiaDR2_RV_star_distance.csv.gz. The rest of the relevant datasets and toy models are available from the corresponding author on reasonable request.
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This work made use of data from ESA mission Gaia (https://www.cosmos.esa.int/gaia), which was processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This project received funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement number 745617. This work was supported by the MDM-2014-0369 of ICCUB (Unidad de Excelencia ‘María de Maeztu’) and the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement GENIUS FP7-606740. A.H. acknowledges financial support from a VICI grant from the Netherlands Organisation for Scientific Research (NWO). We acknowledge the MINECO (Spanish Ministry of Economy) through grants ESP2016-80079-C2-1-R (MINECO/FEDER, UE) and ESP2014-55996-C2-1-R (MINECO/FEDER, UE). This work been funded in part by the Agenzia Spaziale Italiana (ASI) through contract 2014-025-R.1.2015 through the Italian Istituto Nazionale di Astrofisica (INAF). E.P. acknowledges the financial support of the 2014 PhD fellowship programme of INAF.