Outer structure of the Galactic warp and flare: Explaining the Canis Major over-density

Abstract

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 A⊙ ≃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 RGC ∼ 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 R⊙ ≃ 3 and 7 kpc rings; (v) a Northern/Southern warp symmetry is observed only for the ring at R⊙ ≃ 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 RGC of the disk thickness (flaring) and derive an almost constant scale-height (∼0.65 kpc) within RGC ∼ 15 kpc. Further out, the disk flaring increase gradually reaching a mean scale-height of ∼ 1.5 kpc at RGC ∼ 23 kpc; (vii) the derived outer disk warping and flaring provide further robust evidence that there is no disk radial truncation at RGC ∼ 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 R⊙ ≃ 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 Hi 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. © ESO 2006.