Warp, Waves, and Wrinkles in the Milky Way

Abstract

We derive unbiased distance estimates for the Gaia-TGAS dataset by correcting for the bias due to the distance dependence of the selection function, which we measure directly from the data. From these distances and proper motions, we estimate the vertical and azimuthal velocities, $W$ and $V_\phi$, and angular momentum $L_z$ for stars in the Galactic centre and anti-centre directions. The resulting mean vertical motion $\overline{W}$ shows a linear increase with both $V_\phi$ and $L_z$ at $10 \sigma$ significance. Such a trend is expected from and consistent with the known Galactic warp. This signal extends to stars with guiding centre radii $R_g<R_0$, placing the onset of the warp at $R\lesssim7{\rm kpc}$. At equally high significance, we detect a previously unknown wave-like pattern of $\overline{W}$ over guiding centre $R_g$ with amplitude $\sim1{\rm kms}^{-1}$ and wavelength $\sim2.5{\rm kpc}$. This pattern is present in both the centre and anti-centre directions, consistent with a winding (corrugated) warp or bending wave, likely related to known features in the outer disc (TriAnd and Monoceros over-densities), and may be caused by the interaction with the Sgr dwarf galaxy $\sim1{\rm Gyr}$ ago. The only significant deviation from this simple fit is a stream-like feature near $R_g\sim9{\rm kpc}$ ($|L_z|\sim2150{\rm kpckms}^{-1}$).