The Sagittarius impact as an architect of spirality and outer rings in the Milky Way


Like many galaxies of its size, the Milky Way is a disk with prominent spiral arms rooted in a central bar1, although our knowledge of its structure and origin is incomplete. Traditional attempts to understand our Galaxy’s morphology assume that it has been unperturbed by major external forces. Here we report simulations of the response of the Milky Way to the infall of the Sagittarius2 dwarf galaxy (Sgr), which results in the formation of spiral arms, influences the central bar and produces a flared outer disk. Two ring-like wrappings emerge towards the Galactic anti-Centre in our model that are reminiscent of the low-latitude arcs observed in the same area of the Milky Way. Previous models have focused on Sgr itself3,4 to reproduce the dwarf’s orbital history and place associated constraints on the shape of the Milky Way gravitational potential, treating the Sgr impact event as a trivial influence on the Galactic disk. Our results show that the Milky Way’s morphology is not purely secular in origin and that low-mass minor mergers predicted to be common throughout the Universe5 probably have a similarly important role in shaping galactic structure.

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Figure 1: Visualizations of evolved disk end states in the simulation suite.
Figure 2: Face-on surface density visualizations of the Milky Way at four important moments during the ‘light Sgr’ simulation.
Figure 3: The observed Sgr tidal debris stream and remnant core in comparison to our ‘light Sgr’ simulation, in equatorial coordinates.
Figure 4: Endstate disk overdensities in the ‘heavy Sgr’ simulation.


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We would like to thank K. Johnston, M. Kaplinghat, D. Law, H. Morrison and A. Zentner for discussions and C. Struck for suggestions that improved this work. All simulations were performed on the GreenPlanet cluster at the University of California, Irvine.

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C.W.P. helped to conceive the project, performed and analysed all simulations, and wrote the majority of the text. J.S.B. helped to conceive the project, contributed to the analysis, and co-authored the text. E.J.T. created the 3D visualizations and provided discussion and direction related to observational correlations. M.R. provided the code that was utilized to initialize the Sagittarius progenitors and insight on how to thereby achieve the desired properties of these systems. S.C. assisted with interpretation and analysis of the stellar disk instabilities and their time evolution.

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Correspondence to Chris W. Purcell.

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Purcell, C., Bullock, J., Tollerud, E. et al. The Sagittarius impact as an architect of spirality and outer rings in the Milky Way. Nature 477, 301–303 (2011).

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