ON the Nature of the Local Spiral Arm of the Milky Way

TL;DR

This study uses precise parallax measurements to analyze the structure and motion of the Milky Way's Local arm, revealing it is a substantial, arm-like structure with specific kinematic properties, likely a branch of a major spiral arm.

Contribution

It provides new, detailed measurements of the Local arm's structure and motions, challenging previous notions and suggesting it is a significant, arm-like feature rather than a short spur.

Findings

The Local arm has a shallow pitch angle (~10 degrees).

It is more closely associated with the Perseus arm than Sagittarius.

The Local arm exhibits kinematic properties similar to other major spiral arms.

Abstract

Trigonometric parallax measurements of nine water masers associated with the Local arm of the Milky Way were carried out as part of the BeSSeL Survey using the VLBA. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and 3-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (>5 kpc), and shallow pitch angle (~10 degrees) suggest that it is more like the adjacent Perseus and Sagittarius arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local arm to be closer to the Perseus than to the Sagittarius arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near-zero toward both the Galactic center and north Galactic pole, and counter rotation of ~ 5 km/s were observed, indicating that the Local arm has similar kinematic properties as found for other major spiral arms.