Arm Tangents and the spiral structure of the Milky Way ; the Age
  Gradient
  From the Sun, a look at the edge of each spiral arm in our Milky Way (seen
tangentially, along the line of sight) can yield numerous insights. Using
different arm tracers (dust, masers, synchrotron emission, CO gas, open star
clusters), we observe here for the first time an age gradient (about 12 +/-2
Myrs/kpc), much as predicted by the density wave theory. This implies that the
arm tracers are leaving the dust lane at a relative speed of about 81 +/-10
km/s. We then compare with recent optical data obtained from the Gaia
satellite, pertaining to the spiral arms.
