On the kinematic signature of the Galactic warp as revealed by the LAMOST-TGAS data

Abstract

Using a sample of about 123,000 stars with accurate 3D velocity measurements from the LAMOST-TGAS data, we confirm the kinematic signature of the Galactic warp found by Schönrich & Dehnen recently. The data reveal a clear trend of increasing mean vertical velocity $\overline{V_{z}}$ as a function of absolute vertical angular momentum $L_{z}$ and azimuthal velocity $V_ϕ$ for guiding center radius $R_{g}$ between 6.0 and 10.5 kpc. The trend is consistent with a large-scale Galactic warp. Similar to Schönrich & Dehnen, we also find a wave-like pattern of $\overline{V_{z}}$ versus $L_{z}$ with an amplitude of $\sim 0.9$ km s$^{-1}$ on a scale of $\sim 2.0$ kpc, which could arise from bending waves or a winding warp. Finally, we confirm a prominent, localized peak in $\overline{V_z}$ near $L_z \sim 2150$ kpc km s$^{-1}$ (corresponding to $R_{g} \sim 9$ kpc and $V_ϕ \sim 255$ km s$^{-1}$). The additional line-of-sight velocity information from LAMOST reveals that stars in this feature have a large, inward radial velocity of $V_{R} \sim -13.33 \pm 0.59$ km s$^{-1}$ and a small radial velocity dispersion $σ_{R} \sim 25.27 \pm 0.89$ km s$^{-1}$, suggesting that a stellar stream gives rise to this feature.