The spiral structure of our Galaxy— A review of current studies

Abstract

A review of present knowledge of the spiral structure of the Galaxy is given, concentrating on work published since 1970. The principal methods of determining spiral structure in the Galaxy are described and the value of different spiral tracers is assessed. From direct mapping, spiral features within about 4 kpc from the Sun can at present be located and in regions of low absorption up to 10 kpc distance. The main features are the inner (−I) arm from Sagittarius to Carina at a distance of 1.7 kpc, the Cygnus (local) arm with the Orion Spur, the Perseus (+I) arm at a distance of 2.4 kpc, and the arm −II at 4 kpc distance. Wolf-Rayet stars and stellar rings are the only far-reaching optical tracers. The structure indicated by the Wolf-Rayet stars, as based on a revised luminosity calibration, agrees very well with that given by the stellar rings, suggesting a late Sb galaxy. All-sky photographs of the Milky Way in the colours UBVR yield the classification Sb(−b +) I-II. Star polarizations as well as characteristics of dark nebulae (studied on all-sky photographs) are predominantly due to rather local structure. The kinematic approach to deriving spiral structure is plagued by the difference in the kinematics of the young stars and of the gas, and the problem of crowding in velocity space. The second part of this review deals with the internal structure of spiral features. The extension normal to the plane is very considerable. A wave-like structure in the Z-dimension (“shingles”) with about 1.4 kpc wavelength and only 70 pc thickness is observed. It can be quantitatively explained as a density wave normal to the plane, excited by a two-stream instability due to the relative flow of disc and extreme population I stars. In the third part some remarks have been made supplementing this review by considering theoretical possibilities to explain spiral structure apart from the density-wave theory, and summarizing some facts of observation left unexplained by the latter. In addition, various observations present strong evidence for expanding motions in our Galaxy, from about 130 km/sec at 0.3 kpc to about 15 km/sec at 5 kpc from the centre. A density-wave theory using the long-wave mode is sketched. It gives a better fit than the short-wave mode to the optical and to Weaver's HI data, and allows transport of energy and momentum from the inner regions since the wave is outward-going from the material arms which are formed as trailing synchrones by continuous ejection from the nucleus.