The Observed Age Gradient in the Milky Way -- as a Test for theories of
  spiral arm structure
  Some important predictions from 4 main models of spiral arm formation are
tested here, using observational data acquired for the Milky Way galaxy. Many
spiral arm models (density wave, tidal wave, nuclear Lyapunov tube, or dynamic
transient wave) have some consistencies with some of the observations, and some
inconsistencies. Our 4 tests consist of the relative locations and relative
speeds of different arm tracers away from the dust lane, and the global arm
pitch angle as obtained over two Galactic quadrants and several Galactic radii,
as well as the arm's continuity of shape from Galactic quadrant IV to Galactic
quadrant I. In the Milky Way, an age gradient is observed from different arm
tracers, amounting to 12.9 +/-1.1 Myrs/kpc, or a relative speed away from the
dust lane of 76 +/-10 km/s. The presence of an age gradient is predicted by the
density waves, but is not consistent with the predictions of the tidal waves,
of the nuclear Lyapunov tubes, nor of the dynamic transient recurrent waves.
