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# A dynamically young and perturbed Milky Way disk

## Abstract

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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## Data availability

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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## Acknowledgements

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.

## Author information

Authors

### Contributions

T.A. contributed to the sample preparation, analysed and interpreted the data, performed most of the modelling and wrote the paper together with A.H. A.H. also provided interpretation of the findings. M.R.-G. performed the simulation with the barred potential and contributed to sample preparation. D.K., C.B., R.D., D.W.E., F.F., E.P., C.R., A.C.R., G.S. and C.S. contributed to sample preparation and validation of the Gaia data. All authors reviewed the manuscript.

### Corresponding author

Correspondence to T. Antoja.

## Ethics declarations

### Competing interests

: The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Velocities of the stars at the solar Galactocentric radius.

Two-dimensional histograms of combinations of radial, azimuthal and vertical Galactic cylindrical velocities for the stars in our sample of Gaia data located at 8.24 kpc < R < 8.44 kpc, in bins of 1 km s−1. VR and Vϕ are positive towards the Galactic anticentre and the direction of Galactic rotation, respectively. The darkness is proportional to the number of counts. a, Although the bimodality seen here, separating the Hercules stream from the rest of the distribution, was known45,46, as well as some other elongated structures in this velocity projection11,47,48, the numerous and thin arches are a new phenomenon revealed by Gaia data12. The semi-circular dotted line marks an arbitrary line of constant kinetic energy in the plane $${E}_{k}=({V}_{R}^{2}+{V}_{\phi }^{2})/2$$, as predicted for the substructure generated in horizontal phase mixing7,8. b, The data have a box-like appearance, where the extent in VZ of the arches varies with Vϕ (arrows), probably created by the correlation between the spiral shape and Vϕ seen in Fig. 1c. c, Although some velocity asymmetries where noticed before in the VϕVZ projection11 and attributed to the Galaxy warp, the sharp shell-like features involving VZ, especially at VZ ≈ −30 km s−1 and VZ ≈ 25 km s−1, were not previously evident. These shells are different projections of the snail shell pattern of Fig. 1a.

### Extended Data Fig. 2 Location of the stars in the sample.

a, b, Two-dimensional histograms with bins of 0.05 kpc in the XY (a) and XZ (b) projections of our sample of Gaia data. The dotted lines mark the selection of stars in the solar Galactic ring between radii of 8.24 kpc and 8.44 kpc. The Sun is located at (X, Y, Z) = (−8.34, 0, 0.027) kpc and the Galactic centre (GC) is marked with a black dot.

### Extended Data Fig. 3 Modelled vertical positions and velocities of stars with time.

The plots show the snail shells created in the phase space evolution under an anharmonic potential. ac, Phase-space evolution at different times (t = 0, 10, 100, 200, 1,000 Myr) for an ensemble of particles at a fixed Galactocentric radius of R = 8.5 kpc with an initial Gaussian distribution in Z(t = 0) with mean of −0.1 kpc and dispersion of 0.04 kpc and in VZ(t = 0) with mean of −2 km s−1 and dispersion of 1 km s−1. df, Same as ac, but for a skewed normal distribution of initial radius with skewness of 10, location parameter of 8.4 kpc and scale parameter of 0.2 kpc. In all cases, the evolution is under an anharmonic oscillator derived from the expansion of a Miyamoto–Nagai disk for small Z. In a and d the stars are colour-coded by vertical period.

### Extended Data Fig. 4 Vertical frequency for orbits in a Galaxy model.

a, b, Frequencies as a function of Galactocentric radius R computed in the updated model from ref. 41, colour coded by the vertical amplitude (a) and by the vertical velocity amplitude (b) of the orbits.

### Extended Data Fig. 5 Position of the spiral turns in the vertical positions and velocities.

The ZVZ plane for stars at Galactocentric radii of 8.24 kpc to 8.44 kpc, coloured as a function of median guiding radius Rg in bins of ΔZ = 0.04 kpc and ΔVZ = 2 km s−1, with vertical and horizontal lines showing the approximate locations of the observed snail shell (turn-around and mid-plane points).

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Antoja, T., Helmi, A., Romero-Gómez, M. et al. A dynamically young and perturbed Milky Way disk. Nature 561, 360–362 (2018). https://doi.org/10.1038/s41586-018-0510-7

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### Keywords

• Sagittarius Dwarf Galaxy
• Snail Shells
• Diagonal Ridge
• Vertical Frequency
• Skew-normal Distribution

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