2006MNRAS.366..865B

The recently discovered stellar system in Canis Major is analysed using He-burning red clump stars as tracers. Canis Major turns out to be the strongest and the most spatially confined overdensity of the whole Galactic disc, in terms of both number density and statistical significance. When projected on to the Galactic plane, it appears as an elongated and compact overdensity extending from l∼ 200° to 280° with a roundish core towards l∼ 240°. We find that the main body of the system has an integrated absolute magnitude M_V=-14.4±0.8, a central surface brightness µ_V,0_≃ 24.0±0.6 and a line-of-sight (LOS) profile peaked at D_☉= 7.2±1.0kpc with half width at half maximum ∼ 2.0kpc, in excellent agreement with the results obtained with widely different tracers (M giants and main-sequence stars) in previous analyses. The mean distance to the main body of Canis Major is observed to increase with increasing Galactic longitude, from D_☉≃ 6.3kpc at l≃ 225° to D_☉≃ 9.3kpc at l≃ 265°, in good agreement with the predictions of our more recent N-body simulation that models Canis Major (CMa) as a dwarf galaxy being accreted in a planar orbit on to the disc of the Milky Way. We confirm that the Canis Major system has all the characteristics of the relic of a dwarf galaxy seen on top of a large-scale overdensity that we detect all over the third and fourth Galactic quadrants ( 180°≤l≤ 360°, with a strong maximum around l= 290° and b≳-5°) that is identified as the stellar component of the Southern Galactic warp. On the other hand, the possibility that a peculiar deformation/asymmetry of the outer Galactic disc may be at the origin of the observed distribution of overdensities towards CMa cannot be definitely ruled out with the data presented in this paper. We also address a recent claim that Canis Major is on the outskirts of a larger `Argo' structure centred at l≃ 290°. Our analysis shows that the stellar populations in the latter are distributed over a very large distance range along the LOS, and do not give rise to a narrow peak in density in this direction, contrary to what is observed in Canis Major. This suggests that the Argo structure is likely due to Galactic asymmetries such as the warp.