3D self-consistent N-body barred models of the Milky Way: I. Stellar dynamics

Abstract

Many 3D N-body barred models of the Galaxy extending beyond the Solar circle are realised by self-consistent evolution of various bar unstable axisymmetric models. The COBE/DIRBE K-band map, corrected for extinction, is used to constrain the location of the observer in these models, assuming a constant mass-to-light ratio. The resulting view points in the best matching models suggest that the inclination angle of the Galactic bar relative to the Sun-Galactic centre line is 28+-7 degree. Scaling the masses according to the observed radial velocity dispersion of M giants in Baade's Window, several models reproduce satisfactorily the kinematics of disc and halo stars in the Solar neighbourhood, as well as the disc local surface density and scale parameters. These models have a face-on bar axis ratio b/a=0.5+-0.1 and a bar pattern speed Omega_p=50+-5 km/s/kpc, corresponding to a corotation radius of 4.3+-0.5 kpc. The HI terminal velocity constraints favour models with low disc mass fraction near the centre. The large microlensing optical depths observed towards the Galactic bulge exclude models with a disc scale height hz<250 pc around R=4 kpc, arguing for a constant thickness Galactic disc. The models also indicate that a spiral arm starting at the near end of the bar can contribute as much as 0.5x10^(-6) to the optical depth in Baade's Window. The mass-to-K luminosity ratio of the Galactic bulge is probably more than 0.7 (Solar units), and if the same ratio applies outside the bar region, then the Milky Way should have a maximum disc.