The Monoceros Ring is a long, complex, ringlike filament of stars that wraps around the Milky Way three times. This is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy by tidal forces as part of the process of merging with the Milky Way over a period of billions of years, although this view has long been disputed (see Dispute section) The ring contains 100 million solar masses and is 200,000 light years long.
The stream of stars was first reported in 2002 by astronomers conducting the Sloan Digital Sky Survey. It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the Canis Major Dwarf Galaxy was discovered.
In 2006, a study using 2MASS data cast doubts on the nature of the "Ring", arguing that the data suggests that the ring is actually part of the warped galactic disc of the Milky Way. However, observations using the Anglo-Australian Telescope published in 2007 suggest that a warped disc cannot create the observed structure, which must therefore be formed either by a flare of the Galactic Disc or have an extra-Galactic origin.
Several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.
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- Y. Momany et al. (2006). "Outer structure of the Galactic warp and flare: explaining the Canis Major over-density" (PDF). Astronomy and Astrophysics 451 (2): 515–38. arXiv:astro-ph/0603385. Bibcode:2006A&A...451..515M. doi:10.1051/0004-6361:20054081.
- M. Lopez-Corredoira, A; et al. (July 2012). "Comments on the "Monoceros" affair". arXiv:1207.2749.
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- Comments on the "Monoceros" affair, Lopez-Corredoira, M., et al., 2012, arXiv:1207.2749