A Kinematically Cold Structure of Candidate Young OB Stars toward the Anticenter.

We combine Galaxy Evolution Explorer 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°, above the plane up to ~700 pc and below the plane to ~1 kpc. The bulk motion is disklike; however, we measure a mean upward vertical motion of 5.7 ± 0.4 km s-1, and a mean outward radial motion of between 8 and 16 km s-1. The velocity dispersion along the least dispersed of its proper-motion axes (perpendicular to the Galactic disk) is 6.0 ± 0.3 km s-1, 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.