On the Spiral Structure of the Milky Way Galaxy

TL;DR

This paper analyzes the Milky Way's spiral structure using hydrogen distribution data, suggesting a four-arm pattern in the inner region and transient, asymmetric arms in the outer Galaxy, influenced by gravitational instability and magnetic fields.

Contribution

It provides a detailed model of the Galaxy's spiral arms, combining observational data with hypotheses on symmetry and arm formation, highlighting the transient nature of outer arms.

Findings

Inner Galaxy best fits a four-arm spiral pattern.

Outer arms are asymmetric and possibly transient.

Regular magnetic fields are indicated in prominent arms.

Abstract

We consider the possible pattern of the overall spiral structure of the Galaxy, using data on the distribution of neutral (atomic), molecular, and ionized hydrogen, on the base of the hypothesis of the spiral structure being symmetric, i.e. the assumption that spiral arms are translated into each other for a rotation around the galactic center by 180{\deg} (a two-arm pattern) or by 90{\deg} (a four-arm pattern). We demonstrate that, for the inner region, the observations are best represented with a four-arm scheme of the spiral pattern, associated with all-Galaxy spiral density waves. The basic position is that of the Carina arm, reliably determined from distances to HII regions and from HI and H2 radial velocities. This pattern is continued in the quadrants III and IV with weak outer HI arms; from their morphology, the Galaxy should be considered an asymmetric multi-arm spiral. The kneed shape of the outer arms that consist of straight segments can indicate that these arms are transient formations that appeared due to a gravitational instability in the gas disk. The distances between HI superclouds in the two arms that are the brightest in neutral hydrogen, the Carina arm and the Cygnus (Outer) arm, concentrate to two values, permitting to assume the presence of a regular magnetic field in these arms.