On the spiral structure of the Milky Way Galaxy

We consider a possible scheme of the overall spiral structure of the Galaxy using data on the distribution of neutral (atomic), molecular, and ionized hydrogen. Our analysis assumes that the spiral structure is symmetric, i.e., that the spiral arms are translated into each other via a rotation about the Galactic center by 180° (a two-arm pattern) or 90° (a four-arm pattern). In the inner region, the observations are best represented with a four-arm spiral pattern, associated with all-Galaxy spiral density waves. The initial position is that of the Carina arm, reliably determined from distances to HII regions and from HI and H2 radial velocities. This pattern continues in quadrants III and IV with weak outer HI arms; from their morphology, the Galaxy should be considered an asymmetric multi-arm spiral. The knee-like shape of the outer arms, which consist of straight segments, may indicate that these arms are transient formations that appeared due to gravitational instability in the gaseous disk. The distances between HI superclouds in the two arms that are brightest in neutral hydrogen, the Carina and Cygnus (Outer) arms, are concentrated near two values, suggesting the presence of a regular magnetic field in these arms.