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

**Authors:** J. E. G. Peek, Kirill Tchernyshyov, Marc-Antoine Miville-Deschenes  
**arXiv:** [2112.07677](https://arxiv.org/abs/2112.07677)  
**Date:** 2021-12-14  
**Summary page:** https://www.summarizepaper.com/en/paper/2112.07677v1/

## Key points

- The authors analyze the Perseus Arm in the second quadrant of the Milky Way galaxy
- They investigate whether it is a continuous structure or an illusory spiral arm
- Velocity structures in the outer disk can create misleading interpretations of spiral arm features
- They measure distances to CO clouds and find they do not preferentially lie at the distance associated with the Perseus Arm
- Some evidence suggests a closer high pitch angle structure located between 1 and 1.5 kpc away
- Velocity perturbations near the Perseus Arm can impact our interpretation of longitude-velocity diagrams for more than half of the Milky Way disk
- No clear evidence for shallow pitch angle spiral structures in G19 dust maps
- Kinetic tomography maps confirm previous findings of bulk flows of material in the galaxy

## Summary

In this work, the authors present a refined and expanded analysis of the Perseus Arm in the second quadrant of the Milky Way galaxy. They aim to investigate whether the Perseus Arm is a continuous structure of molecular gas or if it is an illusory spiral arm created by the velocity structure of the outer disk. To address this question, they first explore the possibility that observed, distance-resolved velocity structures in the outer disk can create illusory spiral arms, as proposed by Burton in 1971. They demonstrate that these velocity structures have the potential to generate misleading interpretations of spiral arm features. Next, they focus on measuring distances to a collection of CO clouds that exhibit velocities consistent with those expected for the Perseus Arm within a specific range of galactic longitudes ($135^\circ < l < 160^\circ$). To determine these distances, they utilize 3D dust maps from Green et al. (2019). The authors find that these molecular clouds do not preferentially lie at the distance associated with the purported Perseus Arm. Instead, they extend over a distance of approximately 3 kiloparsecs (kpc), with some evidence suggesting a closer high pitch angle structure located between 1 and 1.5 kpc away. Finally, the authors investigate how velocity perturbations near the Perseus Arm can impact our interpretation of longitude-velocity diagrams for more than half of the Milky Way disk. They demonstrate that such perturbations can significantly affect our understanding of galactic dynamics and complicate interpretations based on these diagrams. The authors also discuss previous findings by G19 regarding associations between Reid14 high-mass star-forming regions and regions of dense dust. They note that there is no clear evidence for shallow pitch angle spiral structures in G19 dust maps themselves, which aligns with their own findings. To support their analysis, they utilize kinetic tomography maps published in TPZ18 which provide a comprehensive view of the velocity field across the Milky Way disk. These maps confirm previous findings of bulk flows of material in the galaxy further emphasizing its complexity and dynamic nature.