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

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 $12.9 \pm 1.1$ 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.

Kinematic Structure of the Milky Way Galaxy, Near the Spiral Arm Tangents

“…This global arm pitch angle was found earlier using a fit of arm segments well over both Galactic quadrants I and IV, enabling better precision (Table 1 in [9]; Tables 1 and 2 in [10]; Fig. 4 in [7]); small localised pitch deviations along the Galactic radius are thus smoothed out (see Fig. 1 in [11]).…”

“…We published a 4-arm spiral model as fitted to the tangent in broad diffuse CO gas peaking in each spiral arm (see [5] [6], for the basic equations). A later fit [7] was done, with more data and with improved Galactic parameters: 8.15 kpc for the Sun's distance to the Galactic Center [8]. Other arm parameters are the arm pitch angle = 13.1˚, and the arms start at 2.2 kpc from the Galactic Center.…”

Superposing the Magnetic Spiral Structure of the Milky Way, on the Stellar Spiral Arms—Matching the Unique Galactic Magnetic Field Reversal Zone with Two Galactic Spiral Arm Segments

“…We used a Sun-GC distance of 8.15 kpc, a log-spiral arm shape, arm pitch of 13.4˚ (Fig. 4 in [4]), each arm starting at 2.2 kpc from the Galactic Center [21]. This Galactic-wide model is basic; here we are not interested in fitting local deviations or small-scale structures (supernova explosion, HII region bubble, Local Arm(let), and clumps or voids).…”

“…We are still searching for the answers to four predictions (above): location of peak synchrotron inside the arm width, proper width and location of a reversed magnetic annulus, implication of magnetism for a typical spiral arm, magnetic effects of interstellar turbulence on the regular magnetic field (clockwise or counterclockwise). 4…”

“…In the Milky Way, arm tangents as observed in different tracers have shown offsets from one another (see [3] [4]), with the dust at the inner arm edge, followed by masers, then young HII regions. Before reaching a spiral arm, the gas, clouds, stars all move at the circular orbital speed around the Galactic Center at about 233 km/s as observed [5].…”