Burton's Curse: The Impact of Bulk Flows on the Galactic Longitude-Velocity Diagram and the Illusion of a Continuous Perseus Arm

Abstract

In this work, we demonstrate that the Perseus Arm is not a continuous structure of molecular gas in the second quadrant. We first show that the observed, distanced-resolved velocity structure of the Galaxy in the outer disk is capable of creating illusory spiral arms, as was first proposed by Burton. Second, we measure the distances to a collection of CO clouds at velocities consistent with the Perseus arm with 135° < l < 160°. We find these distances using 3D dust maps from Green et al. We determine that these molecular clouds do not preferentially lie at the distance of a purported Perseus arm but rather extend over 3 kpc in distance, with some evidence for a closer, high pitch angle structure between 1 and 1.5 kpc away. Finally, we demonstrate that velocity perturbations of the amplitude found near the Perseus arm can wreak havoc on our interpretation of the longitude-velocity diagram for more than half of the Milky Way disk.