A Map of the Local Velocity Substructure in the Milky Way Disk

TL;DR

This study maps and quantifies local velocity substructures in the Milky Way disk within 2 kpc of the Sun, revealing significant deviations and streaming motions using corrected stellar velocities from LAMOST data.

Contribution

It provides the first detailed, high-resolution map of velocity substructures in the Milky Way disk near the Sun, with systematic proper motion corrections and new insights into disk kinematics.

Findings

Significant velocity deviations of 20 km/s or more in the disk.

Velocity variations occur over half-kpc scales.

Disk rotation speed increases by 20 km/s outside the solar circle.

Abstract

We confirm, quantify, and provide a table of the coherent velocity substructure of the Milky Way disk within 2 kpc of the Sun towards the Galactic anticenter, with 0.2 kpc resolution. We use the radial velocities of ~340,000 F-type stars obtained with the Guoshoujing Telescope (also known as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, LAMOST), and proper motions derived from the PPMXL catalog. The PPMXL proper motions have been corrected to remove systematic errors by subtracting the average proper motions of galaxies and QSOs that have been confirmed in the LAMOST spectroscopic survey, and that are within 2.5 degrees of the star's position. We provide the resulting table of systematic offsets derived from the PPMXL proper motion measurements of extragalactic objects identified in the LAMOST spectroscopic survey. Using the corrected phase- space stellar sample, we find statistically significant deviations in the bulk disk velocity of 20 km/s or more in the three dimensional velocities of Galactic disk stars. The bulk velocity varies significantly over length scales of half a kpc or less. The rotation velocity of the disk increases by 20 km/s from the Sun's position to 1.5 kpc outside the solar circle. Disk stars in the second quadrant, within 1 kpc of the Sun, are moving radially towards the Galactic center and vertically towards a point a few tenths of a kpc above the Galactic plane; looking down on the disk, the stars appear to move in a circular streaming motion with a radius of order 1 kpc.