Outer structure of the Galactic warp and flare: explaining the Canis Major overdensity

Aims.In this paper we derive the structure of the Galactic stellar warp and flare. Methods: .We use 2MASS red clump and red giant stars, selected at mean and fixed heliocentric distances of Rsun≃3, 7 and 17 kpc. Results: .Our results can be summarized as follows: (i) a clear stellar warp signature is derived for the 3 selected rings, proving that the warp starts already within the solar circle; (ii) the derived stellar warp is consistent (both in amplitude and phase-angle) with that for the Galactic interstellar dust and neutral atomic hydrogen; (iii) the consistency and regularity of the stellar-gaseous warp is traced out to about R_GC˜20 kpc; (iv) the Sun seems not to fall on the line of nodes. The stellar warp phase-angle orientation (φ˜15°) is close to the orientation angle of the Galactic bar and this, most importantly, produces an asymmetric warp for the inner Rsun≃3 and 7 kpc rings; (v) a Northern/Southern warp symmetry is observed only for the ring at Rsun≃17 kpc, at which the dependency on φ is weakened; (vi) treating a mixture of thin and thick disk stellar populations, we trace the variation with R_GC of the disk thickness (flaring) and derive an almost constant scale-height (~0.65 kpc) within R_GC˜15 kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ~1.5 kpc at R_GC˜23 kpc; (vii) the derived outer disk warping and flaring provide further robust evidence that there is no disk radial truncation at R_GC˜14 kpc. Conclusions: .In the particular case of the Canis Major (CMa) over-density we confirm its coincidence with the Southern stellar maximum warp occurring near l˜240° (for Rsun≃7 kpc) which brings down the Milky Way mid-plane by ~3° in this direction. The regularity and consistency of the stellar, gaseous and dust warp argues strongly against a recent merger scenario for Canis Major. We present evidence to conclude that all observed parameters (e.g. number density, radial velocities, proper motion etc) of CMa are consistent with it being a normal Milky Way outer-disk population, thereby leaving no justification for more complex interpretations of its origin. The present analysis or outer disk structure does not provide a conclusive test of the structure or origin of the Monoceros Ring. Nevertheless, we show that a warped flared Milky Way contributes significantly at the locations of the Monoceros Ring. Comparison of outer Milky Way H I and CO properties with those of other galaxies favors the suggestion that complex structures close to planar in outer disks are common, and are a natural aspect of warped and flaring disks.