Metastudy of the spiral structure of our home Galaxy

The current maps of the Milky Way disk still have large differences, much like early maps of the Earth's continents made in the 16th century had sizeable differences in the locations of continents and many areas labeled terra incognita. Exactly where are the spiral arms in our home Galaxy (in radius and longitude)? Here a meta-analysis is made of the recent (1995-2001) observational data on the pitch angle (p) and the number (m) of spiral arms in our home Galaxy. In order to clarify our image of the structure of the Milky Way, logarithmic model arms of the form ln (r/r(0)) = k(theta - theta(0)) are fitted to the observed tangents to the spiral arms and to the observed position angle (P.A.) of the Galaxy's central bar. The main results are that p = 12degrees inward and m = 4, with logarithmic spiral arm parameters r(0) = 2.3 kpc and theta(0) = 0degrees for the Norma arm. The value of theta(0) for the other three arms is modeled by rotating the Norma arm in steps of 90degrees. These values are similar to those found by Ortiz & Lepine using earlier observational data, with some differences. The best model predicts an interarm distance near the Sun of S = 2.5 kpc (from the Sagittarius to the Perseus arm) and a distance from the Sun to the Sagittarius arm of 0.9 kpc. These values are compared to our limited and uncertain data from the observed nearby spiral arms. These predicted values near the Sun differ substantially from the predictions of Ortiz & Lepine, as discussed in the text.