# A Kinematically Cold Structure of Candidate Young OB Stars Toward The Anticenter

## TL;DR

This study identifies a kinematically cold, young OB star structure in the Galaxy's outskirts, likely caused by a satellite-induced wobble, providing insights into galactic disk perturbations.

## Contribution

First detection of a young, cold OB star structure in the Galaxy's outer regions, suggesting a satellite-induced disk wobble rather than a spiral arm or warp.

## Key findings

- The structure contains ~300 OB candidates between 12-15 kpc from the Galactic center.
- It exhibits a mean upward vertical motion of 5.7 km/s and outward radial motion of 8-16 km/s.
- Velocity dispersion confirms the young age of the structure.

## Abstract

We combine GALEX and Gaia DR2 catalogs to track star formation in the outskirts of our Galaxy. Using photometry, proper motions and parallaxes we identify a structure of ~ 300 OB-type candidates located between 12 and 15 kpc from the Galactic center that are kinematically cold. The structure is located between l = 120 and 200 degrees, above the plane up to ~700 pc and below the plane to ~ 1 kpc. The bulk motion is disk-like; however we measure a mean upward vertical motion of 5.7 +-0.4 km/s, and a mean outward radial motion of between 8 and 16 km/s. The velocity dispersion along the least dispersed of its proper-motion axes (perpendicular to the Galactic disk) is 6.0 +-0.3 km/s confirming the young age of this structure. While spatially encompassing the outer spiral arm of the Galaxy, this structure is not a spiral arm. Its explanation as the Milky-Way warp is equally unsatisfactory. The structure's vertical extent, mean kinematics and asymmetry with respect to the plane indicate that its origin is more akin to a wobble generated by a massive satellite perturbing the Galaxy's disk. The mean stellar ages in this outer structure indicate the event took place some 200 Myr ago.