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

TL;DR

This study challenges the traditional view of the Perseus Arm as a continuous structure, showing that apparent spiral arms can be illusions caused by velocity structures and that the actual molecular clouds are more dispersed.

Contribution

The paper provides new distance measurements to CO clouds, revealing the non-continuous nature of the Perseus Arm and highlighting the impact of velocity perturbations on galactic structure interpretation.

Findings

Perseus Arm is not a continuous molecular structure.

Velocity perturbations can create illusions of spiral arms.

Molecular clouds in the Perseus region are spread over 3 kpc.

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 (1971). Second, we measure the distances to a collection of CO clouds at velocities consistent with the Perseus arm with $135^\circ < l < 160^\circ$. We find these distances using 3D dust maps from Green et al. (2019). We determine that these molecular cloud 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.