A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar overdensity, close to the Galactic plane at (l = 240degrees, b = -8degrees), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, 1904, 2298 and 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this overdensity is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disc. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits the extant position, distance and velocity information on the Galactic 'Ring', as well as that of the M-giant overdensities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic plane, with a pericentre at approximately the solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disc, as well as a vertical scaleheight similar to that of the thick disc. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disc, that the thick disc is continually growing, even up to the present time, and that thick disc globular clusters were accreted on to the Milky Way from dwarf galaxies in co-planar orbits.