Galactic Spiral Structure

TL;DR

This paper investigates the structure and dynamics of the Milky Way's spiral arms, proposing a symmetric two-armed model with stable stellar orbits, and suggests the galaxy evolved into this form approximately 9 billion years ago.

Contribution

It introduces a new model of spiral arm motions based on perturbed elliptical stellar orbits and re-examines the Milky Way's spiral structure, challenging previous assumptions.

Findings

Almost all local stars belong to six major streams.

The Milky Way likely has a symmetric two-armed spiral with a smaller pitch angle.

The galaxy evolved into a grand design two-armed spiral about 9 billion years ago.

Abstract

We describe the structure and composition of six major stellar streams in a population of 20 574 local stars in the New Hipparcos Reduction with known radial velocities. We find that, once fast moving stars are excluded, almost all stars belong to one of these streams. The results of our investigation have lead us to re-examine the hydrogen maps of the Milky Way, from which we identify the possibility of a symmetric two-armed spiral with half the conventionally accepted pitch angle. We describe a model of spiral arm motions which matches the observed velocities and composition of the six major streams, as well as the observed velocities of the Hyades and Praesepe clusters at the extreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned at a focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiral arm just before apocentre, follow the arm for more than half an orbit, and leave the arm soon after pericentre. Spiral pattern speed equals the mean rate of precession of apocentre. Spiral arms are shown to be stable configurations of stellar orbits, up to the formation of a bar and/or ring. Pitch angle is directly related to the distribution of orbital eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve to form bars and rings. We show that orbits of gas clouds are stable only in bisymmetric spirals. We conclude that spiral galaxies evolve toward grand design two-armed spirals. We infer from the velocity distributions that the Milky Way evolved into this form about 9 Gyrs ago.