Mapping the Milky Way with LAMOST – IV. The large Galactic disc extending to 35 kpc

Abstract

ABSTRACT We investigate extent of the disc in both vertical and radial directions by separately analysing the density and azimuthal velocity distributions using 31116 M giant stars selected from LAMOST DR5. We first measure the metallicities, radial velocities, and the distances with precision of 0.26 dex, 5.01 km s−1, and 20.8 per cent, respectively. Then, a Bayesian method is applied to model the distributions of the outer Galaxy including components of the thin, thick discs and the halo, to obtain the scale height and azimuthal velocity distributions for the thin and thick discs as a function of galactocentric distance R. The density profile of the thin disc shows that it extends out to R ∼ 35 kpc, beyond which there are no M giant stars in the halo except for Sagittarius Stream members. By using azimuthal velocity to trace the thin disc out to 26 kpc from the Galactic Centre, we show that both the density profile and azimuthal velocity decrease with Galactocentric distance R. In both the density and azimuthal velocity distributions, there is a significant breakpoint at a galactocentric distance of R ∼ 14 kpc. The scale height distribution shows that the thin disc is significantly flared, consistent with previous works. We also find that there is no significant thick disc (as traced by M giant stars) in the outer region beyond R ∼ 15.5 kpc.