Warp, waves, and wrinkles in the Milky Way

Ralph Schönrich,Walter Dehnen

doi:10.1093/mnras/sty1256

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 $R_g\sim9{\rm kpc}$ ($|L_z|\sim2150{\rm kpckms}^{-1}$).

Highlights

The majority of spiral galaxies have at least some warps in their outer gas disc (Bosma 1978, 1981)
Since βU ≈ −0.02 across the sample, accounting for the radial reflex motion of the Sun explains the small ≈ 0.2 km s−1 difference we found in the overall vertical motion between the two samples
The classic approximation of the Galactic warp, where the Sun is placed close to the line of nodes, and the warp appears as a systematic vertical motion of stars, depending on Lz

Summary

Introduction

The majority of spiral galaxies have at least some warps in their outer gas disc (Bosma 1978, 1981). The Milky Way has a well-known warp both in its outer H i (Burke 1957; Kerr 1957; Weaver 1974) and stellar disc (Djorgovski & Sosin 1989). Several studies indicate that the outer disc structure is likely more complicated than a simple warp. Where the Galactic warp starts is still debated. Dehnen (1998) found the signal of a warp starting outside the Solar annulus from the velocities of Hipparcos stars; given the comparably small number of stars, the feature was barely significant, and debated by Seabroke & Gilmore (2007) Where the Galactic warp starts is still debated. Reyle et al (2009), fitting stellar profiles to 2MASS (Skrutskie et al 2006) data via the Besancon model (Robin et al 2003), place the onset of the Galactic warp at or outside the Solar annulus, whereas Drimmel & Spergel (2001) find the onset of the warp about 1 kpc inside the Solar annulus, formally excluding an onset at or outside the Solar position. Dehnen (1998) found the signal of a warp starting outside the Solar annulus from the velocities of Hipparcos stars; given the comparably small number of stars, the feature was barely significant, and debated by Seabroke & Gilmore (2007)

Objectives

Methods

Discussion

Conclusion