National Research Council of Canada. National Science Infrastructure
Disc galaxy; Galaxy formation; Galaxy: fundamental parameters; Galaxy structure; ISM kinematics and dynamics
Though the galactic density wave theory is over 50 years old and is well known in science, it has been difficult to say whether it fits our own Milky Way disk. Here we show a substructure inside the spiral arms. This substructure is reversing with respect to the Galactic Meridian (longitude zero), and crosscuts of the arms at negative longitudes appear as mirror images of crosscuts of the arms at positive longitudes. Four lanes are delineated: a mid-arm (extended ¹²CO gas at the mid-arm, H i atoms), an in-between offset by about 100 pc (synchrotron, radio recombination lines), an in-between offset by about 200 pc (masers, colder dust), and an inner edge (hotter dust seen in mid-IR and near-IR).