The observed age gradient in the Milky Way—as a test for theories of spiral arm structure

Abstract

AbstractSome 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 $12.9 \pm 1.1$ 12.9 ± 1.1  Myrs/kpc, or a relative speed from the dust lane of $76 \pm 10$ 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.