Meta-analysis from different tracers of the small Local Arm around the Sun—extent, shape, pitch, origin

Abstract

The Sun in not located in a major spiral arm, and sits in a small 'Local Arm' (variously called arm, armlet, blob, branch, bridge, feather, finger, segment, spur, sub-arm, swath, etc). The diversity of names for the 'Local Arm' near the Sun indicates an uncertainty about its shape or pitch or its extent from the Sun in each galactic quadrant, as well as an uncertainty about its origin.Here we extract data about the small 'Local Arm' near the Sun, from the recent observational literature, over many arm tracers, and we use statistics in order to find the local arm's mean extent from the Sun, its possible shape and pitch angle from the direction of galactic longitude 90 o . Employing all tracers, the Local Arm is about 4 kpc long by 2 kpc large. The Sun is within 1 kpc of the center of the local arm. Proposed 'bridges' and 'fingers' are assessed. These bridges to nearby spiral arms and fingers across spiral arms may not reach the nearest spiral arms, owing to kinematic and photometric distance effects.We then compare these statistical results with some predictions from recent models proposed to explain the local arm (perturbations, resonances, density wave, halo supercloud, debris trail from a dwarf galaxy).The least controversial models involve importing materials from elsewhere (halo supercloud, debris trail) as a first step, and to be later deformed in a second step (by the Galaxy's differential rotation into become roughly parallel to spiral arms) and then subjected to ongoing forces (global density waves, local perturbations).

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

“…Young stars in the small Local Arm (an armlet, not a long log-periodic arm) could be born from multiple processes, and the Local Arm has different shapes in different tracers -see Vallée (2018a); Laporte et al (2019). Rather than look at nearby stars, it might be useful to look at specific galactic regions (in galactic longitudes), such as where the line of sight from the Sun is tangent to a long spiral arm, to directly map the region from the inner arm side (with dust) across the spiral arms (to the outer arm side).…”

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

“…The Sun is located in an interarm, in between two long spiral arms (Sagittarius arm and Perseus arm). Other small interarm islands are known within our Galactic disk (Vallée 2018;Vallée 2020). The magnetic field strength there is not known (too few data).…”