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 pm 1.1 Myrs/kpc, or a relative speed from the dust lane of 76 pm 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.
Highlights
Long periodic arms in spiral galaxies are well known, and they exists in our Milky Way disk galaxy—for a review, see Vallée (2017b)
Four main models compete for the formation of long spiral arms: density wave, tidal waves from a passing galaxy, Lyapunov tubes from the Galactic nucleus, dynamic transient waves, etc
To define a spiral arm, some have looked at a single tracer (O-B stars, Open star Clusters, etc.) at different ages to look for a color/age gradient
Summary
Long periodic arms in spiral galaxies are well known, and they exists in our Milky Way disk galaxy—for a review, see Vallée (2017b). A recipe was added to select arm tracers and fitting functions to detect an offset between a starforming region near a ‘shock front’ and a non-starforming region near a ‘potential minimum’ of an arm (see Table 2 and Fig. 11 in Vallée 2020a). As these two different regions of the same spiral arm are separated by about 350 pc, the linear resolution must be better than 70 pc (at the 5-sigma level)
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