The Norma spiral arm: large-scale pitch angle

TL;DR

This study measures the large-scale pitch angle of the Norma spiral arm in the Milky Way using the twin-tangent method, providing new insights into its structure and comparing it with other galactic arms.

Contribution

It offers the first statistical measurement of the Norma arm's pitch angle in two galactic quadrants and revises its velocity plot based on recent data.

Findings

Pitch angle of Norma arm is approximately -13.7 degrees.

Revised velocity plot of Norma arm aligns with other spiral arms.

Comparison with literature shows consistency and refinements in arm structure.

Abstract

In the inner Galaxy, we statistically find the mean pitch angle of the recently mapped Norma arm in two galactic quadrants (observed tangentially at galactic longitudes near l=328 degrees and near l=20 degrees), using the twin-tangent method, and obtain -13.7 +/-1.4 degrees. We compared with other measurements in the literature. Also, using the latest published data on pitch angle and the latest published data on the radial starting point of the four arms (RGal = 2.2 kpc) in each galactic quadrant, a revised velocity plot of the Norma spiral arm is made, along with other spiral arms in the Milky Way, in each Galactic quadrant.