A Sagittarius-induced origin for the Monoceros ring

Authors

  • Léo Michel-Dansac,

    Corresponding author
    1. Centre de Recherche Astrophysique de Lyon, Université de Lyon, Université Lyon 1, Observatoire de Lyon, Ecole Normale Supérieure de Lyon, CNRS, UMR 5574, 9 avenue Charles André, Saint-Genis Laval 69230, France
      E-mail: leo.michel-dansac@univ-lyon1.fr (LM-D); mario@oac.uncor.edu (MGA); jfn@uvic.ca (JFN); msteinmetz@aip.de (MS)
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  • Mario G. Abadi,

    Corresponding author
    1. Instituto de Astronomía Teórica y Experimental (IATE), Observatotio Astronómico de Córdoba and CONICET, Laprida 854 X5000BGR Córdoba, Argentina
      E-mail: leo.michel-dansac@univ-lyon1.fr (LM-D); mario@oac.uncor.edu (MGA); jfn@uvic.ca (JFN); msteinmetz@aip.de (MS)
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  • Julio F. Navarro,

    Corresponding author
    1. Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC V8P 5C2, Canada
      E-mail: leo.michel-dansac@univ-lyon1.fr (LM-D); mario@oac.uncor.edu (MGA); jfn@uvic.ca (JFN); msteinmetz@aip.de (MS)
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  • Matthias Steinmetz

    Corresponding author
    1. Leibniz-Institut für Astrophysic Postdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
      E-mail: leo.michel-dansac@univ-lyon1.fr (LM-D); mario@oac.uncor.edu (MGA); jfn@uvic.ca (JFN); msteinmetz@aip.de (MS)
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E-mail: leo.michel-dansac@univ-lyon1.fr (LM-D); mario@oac.uncor.edu (MGA); jfn@uvic.ca (JFN); msteinmetz@aip.de (MS)

ABSTRACT

The Monoceros ring is a collection of stars in nearly circular orbits at roughly 18 kpc from the Galactic Centre. It may have originated (i) as the response of the disc to perturbations excited by satellite companions or (ii) from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros stars differs from that of disc stars at comparable Galactocentric distances, an observation that disfavours the first scenario. On the other hand, circular orbits are difficult to accommodate in the tidal-disruption scenario, since it requires a satellite which at the time of disruption was itself in a nearly circular orbit. Such satellite could not have formed at the location of the ring and, given its low mass, dynamical friction is unlikely to have played a major role in its orbital evolution. We search cosmological simulations for low-mass satellites in nearly circular orbits and find that they result, almost invariably, from orbital changes induced by collisions with more massive satellites: the radius of the circular orbit thus traces the galactocentric distance of the collision. Interestingly, the Sagittarius dwarf, one of the most luminous satellites of the Milky Way, is in a polar orbit that crosses the Galactic plane at roughly the same Galactocentric distance as Monoceros. We use idealized simulations to demonstrate that an encounter with Sagittarius might well have led to the circularization and subsequent tidal demise of the progenitor of the Monoceros ring.

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