The Isaac Newton Telescope Wide Field Camera survey of the Monoceros Ring: accretion origin or Galactic anomaly?

We present the results of a wide-field camera survey of the stars in the Monoceros Ring, thought to be an additional structure in the Milky Way of unknown origin. Lying roughly in the plane of the Milky Way, this may represent a unique equatorial accretion event which is contributing to the thick disc of the Galaxy. Alternatively, the Monoceros Ring may be a natural part of the disc formation process. With 10 pointings in symmetric pairs above and below the plane of the Galaxy, this survey spans 90° about the equator of the Milky Way. Signatures of the stream of stars were detected in three fields, (l, b) = (118°, +16°) and (150°, +15°) plus a more tentative detection at (150°, -15°). Galactocentric distance estimates to these structures gave ~17, ~17, and ~13 kpc, respectively. The Monoceros Ring seems to be present on both sides of the Galactic plane, in a form different from that of the Galactic warp, suggestive of a tidal origin with streams multiply wrapping the Galaxy. A new model of the stream has shown a strong coincidence with our results and has also provided the opportunity to make several more detections in fields in which the stream is less significant. The confirmed detection at (l, b) = (123°, -19°) at ~14, kpc from the Galactic Centre allows a re-examination revealing a tentative new detection with a Galactocentric distance of ~21 kpc. These detections also lie very close to the newly discovered structure in Triangulum-Andromedae hinting at a link between the two. The remaining six fields are apparently non-detections although in light of these new models, closer inspection reveals tentative structure. With the overdensity of M giant stars in Canis Major being claimed both as a progenitor to the Monoceros Ring and alternatively a manifestation of the Milky Way warp, much is still unknown concerning this structure and its connection to the Monoceros Ring. Further constraints are needed for the numerical simulations to adequately resolve the increasingly complex view of this structure.