Surveying the Monoceros Ring: Locations, Velocities and a Contentious Dwarf

Abstract

In 2002, Newberg et al. [7], using the Sloan Digital Sky Survey (DSS), found an overdensity of stars in the outer reaches of the Milky Way. At 11 kpc from the Sun and covering a wide range of longitudes, this structure was dubbed the Monoceros Ring (MRi). Ibata et al. [5] and Conn et al. [1, 2, 3, 4] have shown that this structure is found around the Galactic Plane and on both sides of the Galactic Disc. Several interpretations have been suggested for this structure ranging from perturbations of the outer Disc to the tidal stream of an accreting dwarf galaxy. To investigate these scenarios, a deep imaging survey around the Galactic plane has been performed with both the INT/WFC and AAT/WFI instruments. The Colour-Magnitude diagrams (CMDs) of these regions are compared with the Besancon synthetic galaxy model, see Figure 1. Importantly, the Monoceros Ring can be seen as a distinct main sequence away from the bulk Milky Way components. A main sequence fiducial has been placed on the detection to estimate the distance. This method is used, as isolating the Red Giant Branch from Milky Way stars is difficult in high stellar density regions of the CMD. Radial velocities of MRi stars have been observed in several fields. Conn et al. [1, 2] show the detection of the stream behind the CMa overdensity. In general, the MRi velocity profile is not well known around the Galactic Plane but importantly though, each detection of the MRi does report a cold population of stars, typically with a velocity dispersion of ∼20 km/s. This is combined with a tight correlation in distance. Together these suggest a tidal stream scenario, however more MRi stars need to be observed to better illustrate this correlation. If the MRi is an in-plane accretion event then searching for its progenitor is an important step in understanding its evolution and impact on the Milky Way. Two independent simulations have placed such a progenitor in the Canis Major region, (Martin et al. [6], Penarrubia et al. [8]). The Galactic Disc warps significantly in Canis Major and this needs to be accounted for when