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A dynamically young and perturbed Milky Way disk


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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Fig. 1: Vertical positions and velocities of the stars.
Fig. 2: Positions and velocities of the stars in the disk plane.
Fig. 3: Models of the phase-space distribution of the Galaxy disk.

Data availability

The datasets used and analysed for this study are derived from data available in the public Gaia archive ( The Bayesian distances for the Gaia sources with radial velocity37are available at The rest of the relevant datasets and toy models are available from the corresponding author on reasonable request.


  1. Sellwood, J. A. & Binney, J. J. Radial mixing in galactic discs. Mon. Not. R. Astron. Soc. 336, 785–796 (2002).

    ADS  Article  Google Scholar 

  2. Contopoulos, G. & Grosbol, P. Stellar dynamics of spiral galaxies: nonlinear effects at the 4/1 resonance. Astron. Astrophys. 155, 11–23 (1986).

    ADS  MATH  Google Scholar 

  3. Quinn, P. J., Hernquist, L. & Fullagar, D. P. Heating of galactic disks by mergers. Astrophys. J. 403, 74–93 (1993).

    ADS  Article  Google Scholar 

  4. Purcell, C. W., Bullock, J. S., Tollerud, E. J., Rocha, M. & Chakrabarti, S. The Sagittarius impact as an architect of spirality and outer rings in the Milky Way. Nature 477, 301–303 (2011).

    ADS  CAS  Article  Google Scholar 

  5. D’Onghia, E., Madau, P., Vera-Ciro, C., Quillen, A. & Hernquist, L. Excitation of coupled stellar motions in the Galactic disk by orbiting satellites. Astrophys. J. 823, 4 (2016).

    ADS  Article  Google Scholar 

  6. Fux, R. Order and chaos in the local disc stellar kinematics induced by the Galactic bar. Astron. Astrophys. 373, 511–535 (2001).

    ADS  Article  Google Scholar 

  7. Minchev, I. et al. Is the Milky Way ringing? The hunt for high-velocity streams. Mon. Not. R. Astron. Soc. 396, L56–L60 (2009).

    ADS  Article  Google Scholar 

  8. Gómez, F. A., Minchev, I., Villalobos, Á., O’Shea, B. W. & Williams, M. E. K. Signatures of minor mergers in Milky Way like disc kinematics: ringing revisited. Mon. Not. R. Astron. Soc. 419, 2163–2172 (2012).

    ADS  Article  Google Scholar 

  9. de la Vega, A., Quillen, A. C., Carlin, J. L., Chakrabarti, S. & D’Onghia, E. Phase wrapping of epicyclic perturbations in the Wobbly galaxy. Mon. Not. R. Astron. Soc. 454, 933–945 (2015).

    ADS  Article  Google Scholar 

  10. Eggen, O. J. Star streams and Galactic structure. Astron. J. 112, 1595–1613 (1996).

    ADS  CAS  Article  Google Scholar 

  11. Dehnen, W. The distribution of nearby stars in velocity space inferred from HIPPARCOS data. Astron. J. 115, 2384–2396 (1998).

    ADS  Article  Google Scholar 

  12. Gaia Collaboration. Gaia data release 2: mapping the Milky Way disc kinematics. Astron. Astrophys. 616, A11 (2018).

    Article  Google Scholar 

  13. Siebert, A. et al. Detection of a radial velocity gradient in the extended local disc with RAVE. Mon. Not. R. Astron. Soc. 412, 2026–2032 (2011).

    ADS  Article  Google Scholar 

  14. Widrow, L. M., Gardner, S., Yanny, B., Dodelson, S. & Chen, H.-Y. Galactoseismology: discovery of vertical waves in the Galactic disk. Astrophys. J. 750, L41 (2012).

    ADS  Article  Google Scholar 

  15. Schönrich, R. & Dehnen, W. Warp, waves, and wrinkles in the Milky Way. Mon. Not. R. Astron. Soc. 478, 3809–3824 (2018).

    ADS  Article  Google Scholar 

  16. Quillen, A. C. et al. The GALAH survey: stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere and metallicity. Mon. Not. R. Astron. Soc. 478, 228–254 (2018).

    ADS  CAS  Article  Google Scholar 

  17. Gómez, F. A. et al. Signatures of minor mergers in the Milky Way disc – I. The SEGUE stellar sample. Mon. Not. R. Astron. Soc. 423, 3727–3739 (2012).

    ADS  Article  Google Scholar 

  18. Monari, G. et al. Coma Berenices: the first evidence for incomplete vertical phase-mixing in local velocity space with RAVE—confirmed with Gaia DR2. Res. Notes AAS 2, 32 (2018).

    ADS  Article  Google Scholar 

  19. Gaia Collaboration. Gaia data release 2: summary of the contents and survey properties. Astron. Astrophys. 616, A1 (2018).

    Article  Google Scholar 

  20. Tremaine, S. The geometry of phase mixing. Mon. Not. R. Astron. Soc. 307, 877–883 (1999).

    ADS  Article  Google Scholar 

  21. Afshordi, N., Mohayaee, R. & Bertschinger, E. Hierarchical phase space structure of dark matter haloes: tidal debris, caustics, and dark matter annihilation. Phys. Rev. D 79, 083526 (2009).

    ADS  Article  Google Scholar 

  22. Candlish, G. N. et al. Phase mixing due to the Galactic potential: steps in the position and velocity distributions of popped star clusters. Mon. Not. R. Astron. Soc. 437, 3702–3717 (2014).

    ADS  Article  Google Scholar 

  23. Manfredi, G. & Feix, R. M. Theory and simulation of classical and quantum echoes. Phys. Rev. E 53, 6460–6470 (1996).

    ADS  CAS  Article  Google Scholar 

  24. Law, D. R. & Majewski, S. R. The Sagittarius dwarf galaxy: a model for evolution in a triaxial Milky Way halo. Astrophys. J. 714, 229–254 (2010).

    ADS  Article  Google Scholar 

  25. Laporte, C. F. P., Johnston, K. V., Gómez, F. A., Garavito-Camargo, N. & Besla, G. The influence of Sagittarius and the Large Magellanic Cloud on the Milky Way galaxy. Mon. Not. R. Astron. Soc. (2018).

    ADS  CAS  Article  Google Scholar 

  26. Monari, G., Kawata, D., Hunt, J. A. S. & Famaey, B. Tracing the Hercules stream with Gaia and LAMOST: new evidence for a fast bar in the Milky Way. Mon. Not. R. Astron. Soc. 466, L113–L117 (2017).

    ADS  Article  Google Scholar 

  27. Michtchenko, T. A., Lépine, J. R. D., Barros, D. A. & Vieira, R. S. S. Combined dynamical effects of the bar and spiral arms in a Galaxy model. Application to the solar neighbourhood. Astron. Astrophys. 615, A10 (2018).

    ADS  Article  Google Scholar 

  28. Fouvry, J.-B., Binney, J. & Pichon, C. Self-gravity, resonances, and orbital diffusion in stellar disks. Astrophys. J. 806, 117 (2015).

    ADS  Article  Google Scholar 

  29. Luri, X. et al. Gaia data release 2: using Gaia parallaxes. Astron. Astrophys. 616, A9 (2018).

    Article  Google Scholar 

  30. Chen, B. et al. Stellar population studies with the SDSS. I. The vertical distribution of stars in the Milky Way. Astrophys. J. 553, 184–197 (2001).

    ADS  Article  Google Scholar 

  31. Reid, M. J. et al. Trigonometric parallaxes of high mass star forming regions: the structure and kinematics of the Milky Way. Astrophys. J. 783, 130 (2014).

    ADS  Article  Google Scholar 

  32. Schönrich, R. Galactic rotation and solar motion from stellar kinematics. Mon. Not. R. Astron. Soc. 427, 274–287 (2012).

    ADS  Article  Google Scholar 

  33. Reid, M. J. & Brunthaler, A. The proper motion of Sagittarius A*. II. The mass of Sagittarius A*. Astrophys. J. 616, 872–884 (2004).

    ADS  CAS  Article  Google Scholar 

  34. Lindegren, L. et al. Gaia data release 2: the astrometric solution. Astron. Astrophys. 616, A2 (2018).

    Article  Google Scholar 

  35. Taylor, M. B. TOPCAT & STIL: Starlink table/VOTable processing software. ASP Conf. Ser. 347, 29–33 (2005).

    ADS  Google Scholar 

  36. Astraatmadja, T. L. & Bailer-Jones, C. A. L. Estimating distances from parallaxes. II. Performance of Bayesian distance estimators on a Gaia-like catalogue. Astrophys. J. 832, 137 (2016).

    ADS  Article  Google Scholar 

  37. McMillan, P. J. Simple distance estimates for Gaia DR2 stars with radial velocities. Res. Notes AAS 2, 51 (2018).

    ADS  Article  Google Scholar 

  38. Binney, J. & Tremaine, S. Galactic Dynamics 2nd edn (Princeton Univ. Press, Princeton, 2008).

    MATH  Google Scholar 

  39. Miyamoto, M. & Nagai, R. Three-dimensional models for the distribution of mass in galaxies. Publ. Astron. Soc. Jpn 27, 533–543 (1975).

    ADS  Google Scholar 

  40. Allen, C. & Santillan, A. An improved model of the galactic mass distribution for orbit computations. Rev. Mex. Astron. Astrofis. 22, 255–263 (1991).

    ADS  Google Scholar 

  41. Irrgang, A., Wilcox, B., Tucker, E. & Schiefelbein, L. Milky Way mass models for orbit calculations. Astron. Astrophys. 549, A137 (2013).

    ADS  Article  Google Scholar 

  42. McMillan, P. J. The mass distribution and gravitational potential of the Milky Way. Mon. Not. R. Astron. Soc. 465, 76–94 (2017).

    ADS  CAS  Article  Google Scholar 

  43. Romero-Gómez, M., Figueras, F., Antoja, T., Abedi, H. & Aguilar, L. The analysis of realistic stellar Gaia mock catalogues – I. Red clump stars as tracers of the central bar. Mon. Not. R. Astron. Soc. 447, 218–233 (2015).

    ADS  Article  Google Scholar 

  44. Ferrers, N. On the potentials of ellipsoids, ellipsoidal shells, elliptic laminae and elliptic rings of variable densities. QJ Pure Appl. Math 14, 1–22 (1877).

    MATH  Google Scholar 

  45. Eggen, O. J. Stellar groups. II. The ζ Herculis, ϵ Indi and 61 Cygni groups of high-velocity stars. Mon. Not. R. Astron. Soc. 118, 154–160 (1958).

    ADS  Article  Google Scholar 

  46. Blaauw, A. Remarks on Local Structure and Kinematics. Symp. IAU 38, 199–204 (1970).

    ADS  Google Scholar 

  47. Skuljan, J., Hearnshaw, J. B. & Cottrell, P. L. Velocity distribution of stars in the solar neighbourhood. Mon. Not. R. Astron. Soc. 308, 731–740 (1999).

    ADS  Article  Google Scholar 

  48. Antoja, T., Figueras, F., Fernández, D. & Torra, J. Origin and evolution of moving groups. I. Characterization in the observational kinematic-age-metallicity space. Astron. Astrophys. 490, 135–150 (2008).

    ADS  CAS  Article  Google Scholar 

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This work made use of data from ESA mission Gaia (, which was processed by the Gaia Data Processing and Analysis Consortium (DPAC; 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.

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Authors and Affiliations



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.

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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).

Extended Data Table 1 Time estimates from the turn-around points of the spiral
Extended Data Table 2 Time estimates from the mid-plane points of the spiral

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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).

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

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