The Myth of the Molecular Ring

TL;DR

This study challenges the traditional view of the Milky Way's molecular ring by showing it is better explained as a spiral arm structure, suggesting the galaxy's features are dominated by spiral density waves rather than a true ring.

Contribution

The paper demonstrates that the molecular ring is more accurately modeled as a spiral arm, providing new insights into the galaxy's structure and the role of spiral density waves.

Findings

A spiral arm fits the molecular ring better than a true ring.

The CO emission can be modeled by a symmetric 2-armed spiral pattern.

The spiral arms originate near the ends of the galactic bar.

Abstract

We investigate the structure of the Milky Way by determining how features in a spatial map correspond to CO features in a velocity map. We examine structures including logarithmic spiral arms, a ring and a bar. We explore the available parameter space, including the pitch angle of the spiral arms, radius of a ring, and rotation curve. We show that surprisingly, a spiral arm provides a better fit to the observed molecular ring than a true ring feature. This is because both a spiral arm, and the observed feature known as the molecular ring, are curved in velocity longitude space. We find that much of the CO emission in the velocity longitude map can be fitted by a nearly symmetric 2 armed spiral pattern. One of the arms corresponds to the molecular ring, whilst the opposite arm naturally reproduces the Perseus arm. Multiple arms also contribute to further emission in the vicinity of the molecular ring and match other observed spiral arms. Whether the Galactic structure consists primarily of two, or several spiral arms, the presence of 2 symmetric logarithmic spirals, which begin in the vicinity of the ends of the bar, suggest a spiral density wave associated with the bar.