Galactic spiral structure

摘要

We describe the structure and composition of six major stellar streams in a populationof 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 ofthese streams. The results of our investigation have led us to re-examine the hydrogenmaps of the Milky Way, from which we identify the possibility of a symmetric two-armedspiral with half the conventionally accepted pitch angle. We describe a model of spiralarm motions that matches the observed velocities and compositions of the six majorstreams, as well as the observed velocities of the Hyades and Praesepe clusters at theextreme of the Hyades stream. We model stellar orbits as perturbed ellipses aligned ata focus in coordinates rotating at the rate of precession of apocentre. Stars join a spiralarm just before apocentre, follow the arm for more than half an orbit, and leave thearm soon after pericentre. Spiral pattern speed equals the mean rate of precession ofapocentre. Spiral arms are shown to be stable configurations of stellar orbits, up to theformation of a bar and/or ring. Pitch angle is directly related to the distribution oforbital eccentricities in a given spiral galaxy. We show how spiral galaxies can evolve toform bars and rings. We show that orbits of gas clouds are stable only in bisymmetricspirals. 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 about9 billion years ago (Ga).