Metastudy of the Spiral Structure of Our Home Galaxy

Abstract

The current maps of the Milky Way disk still have large di †erences, much like early maps of the EarthÏs continents made in the 16th century had sizeable di †erences in the locations of continents and many areas labeled "" terra incognita.ÏÏ Exactly where are the spiral arms in our home Galaxy (in radius and longitude) ? Here a meta-analysis is made of the recent (1995È2001) 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 are Ðtted to the ln (r/r 0) \ k(h [ h 0) observed tangents to the spiral arms and to the observed position angle (P.A.) of the GalaxyÏs central bar. The main results are that p \ 12¡ inward and m \ 4, with logarithmic spiral arm parameters r 0 \ 2.3 kpc and for the Norma arm. The value of for the other three arms is modeled by rotating h 0 \ 0¡ h 0 the Norma arm in steps of 90¡. These values are similar to those found by Ortiz & using earlier Le pine observational data, with some di †erences. 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 di †er substantially from the predictions of Ortiz & as Le pine, discussed in the text.