The Observed Age Gradient in the Milky Way -- as a Test for theories of spiral arm structure

TL;DR

This study tests various spiral arm formation models in the Milky Way by analyzing observational data, finding that the observed age gradient supports density wave theory but contradicts other models.

Contribution

It provides observational tests that distinguish between competing theories of spiral arm formation in our galaxy.

Findings

An observed age gradient of 12.9 +/-1.1 Myrs/kpc in the Milky Way's spiral arms.

The age gradient aligns with density wave predictions.

Other models like tidal waves and transient waves are inconsistent with the observed data.

Abstract

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.