Torus models of the outer disc of the Milky Way using LAMOST survey data

With a sample of 48 161 K giant stars selected from the LAMOST DR 2 catalogue, we construct torus models in a large volume extending, for the first time, from the solar vicinity to a Galactocentric distance of ~20 kpc, reaching the outskirts of the Galactic disc. We show that the kinematics of the K giant stars match conventional models, e.g. as created by Binney in 2012, in the Solar vicinity. However such two-disc models fail if they are extended to the outer regions, even if an additional disc component is utilized. If we loosen constraints in Sun's vicinity, we find that an effective thick disc model could explain the anticentre of the MW. The Large Area Multi-Object Spectroscopic Telescope data imply that the sizes of the Galactic discs are much larger, and that the outer disc is much thicker, than previously thought, or alternatively that the outer structure is not a conventional disc at all. However, the velocity dispersion σ0z of the kinematically thick disc in the best-fitting model is about 80 km s-1 and has a scale parameter Rσ for an exponential distribution function of ~19 kpc. Such a height σ0z is strongly rejected by current measurements in the solar neighbourhood, and thus a model beyond quasi-thermal, two or three thin or thick discs is required. © 2017 The Authors.