The Norma spiral arm: large-scale pitch angle

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.

arXiv comment12 pages, 4 figures, 2 tables; accepted for publication in Astrophysics and Space Science

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• Spatial and velocity offsets of Galactic masers from the centers of spiral arms
• The Observed Age Gradient in the Milky Way -- as a Test for theories of spiral arm structure
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