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The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way


Satellites orbiting disk galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their disks. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disk has only recently been fully mapped by Gaia Data Release 2 (DR2) data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disk. Here, we model Gaia DR2-observed colour–magnitude diagrams to obtain a detailed star formation history of the ~2 kpc bubble around the Sun. It reveals three conspicuous and narrow episodes of enhanced star formation that we can precisely date as having occurred 5.7, 1.9 and 1.0 Gyr ago. The timing of these episodes coincides with proposed Sgr pericentre passages according to (1) orbit simulations, (2) phase space features in the Galactic disk and (3) Sgr stellar content. These findings most probably suggest that Sgr has also been an important actor in the build-up of the stellar mass of the Milky Way disk, with the perturbations from Sgr repeatedly triggering major episodes of star formation.

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Fig. 1: CMD of the ~ 2-kpc bubble around the Sun.
Fig. 2: SFH representative of the ~2-kpc-radius bubble around the Sun.
Fig. 3: Testing the robustness of the SFH recovery with mock stellar populations.
Fig. 4: SFH in the ~2-kpc-radius bubble around the Sun distinguishing between the thin and thick disks.

Data availability

All data analysed in this paper are publicly available from the Gaia DR2 archive ( The datasets containing information not available in public catalogues that is necessary to reproduce this work and the corresponding figures are available as Supplementary Data 1. An explanatory README file is included. Supplementary Data 1 consists of the following files: (1) For Figs. 1 and 2, two examples of randomly selected samples of ~250,000 stars (with different extinction coefficients), together with the solution from the THESTORM code. This includes two tables directly retrieved from the Gaia archive as described in the Methods, supplemented by extinction information on a star-by-star basis. (2) For Fig. 3, six mock population CMDs together with the outputs from THESTORM. (3) Examples of randomly selected samples of 250,000 stars for thin and thick disk stars, together with the solutions from the THESTORM code (for Fig. 4). Other data not included in the above-mentioned link are available from the corresponding author on reasonable request.

Code availability

The code used to interpolate the three-dimensional extinction maps63 can be retrieved from


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We thank K. C. Freeman, C. Bailer-Jones, G. Battaglia, M. Beasley, C. Brook, C. Dalla Vecchia, J. Falcón-Barroso, R. Leaman and I. Pérez for useful discussions. T.R.-L. and C.G. acknowledge financial support through grant numbers (AEI/FEDER, UE) AYA2017-89076-P, AYA2016-77237-C3-1-P (RAVET project) and AYA2015-63810-P, as well as from the Ministerio de Ciencia, Innovación y Universidades (MCIU) through the State Budget and the Consejería de Economía, Industria, Comercio y Conocimiento of the Canary Islands Autonomous Community through the Regional Budget (including IAC project, TRACES). T.R.-L. is supported by a MCIU Juan de la Cierva–Formación grant (FJCI-2016-30342). S.C. acknowledges support from Premiale INAF “MITIC” and grant number AYA2013-42781P from the Ministry of Economy and Competitiveness of Spain; he has also been supported by the INFN (Iniziativa specifica TAsP). We used data from the European Space Agency mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC; see Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research makes use of Python (version 3.6.7,; Matplotlib (version 3.0.0)79, a suite of open-source python modules that provide a framework for creating scientific plots; and Astropy (version 3.0.5), a community-developed core Python package for astronomy80,81.

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The manuscript was written by T.R.-L. and C.G. T.R.-L. and C.G. defined the final samples under analysis and extracted the SFH presented in this work. The software to analyse Gaia DR2 data was written by T.R.-L. and E.J.B. S.C. contributed to the tools used to generate the synthetic CMDs and evolutionary model predictions in the Gaia photometric system. All authors contributed to the interpretation and analysis of the results.

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Correspondence to Tomás Ruiz-Lara.

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Peer review information Nature Astronomy thanks Michele Cignoni and Roger Mor for their contribution to the peer review of this work.

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Ruiz-Lara, T., Gallart, C., Bernard, E.J. et al. The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way. Nat Astron 4, 965–973 (2020).

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