A Magellanic effect on the galaxy.

The shape of the galactic disk has been studied, by means of 21-cm hydrogen-line observations, using the Leiden rotational model (van de Hulst et al. 1954) to deduce distances. The whole of the galactic plane, except for a small 450 sector, centered on longitude 900, can be seen from Sydney. The results of this study were reported to a Radio Astronomy Symposium in Sydney in September last (Kerr 1956). A sweep across the galactic plane at a given longitude and frequency gives the vertical position of the galactic "plane" at a particular distance. A relief map of the galaxy can be built up in this way. When drawn with reference to the conventional galactic plane, the main effect is the well-known downward slope, approximately towards the center. This slope has been found from observations over the nuclear region, leading to a solution for the "principal plane," that is, the equatorial plane of the most massive part of the galaxy. Allowing for this slope, the central part becomes substantially flat, but the outer regions show a twist-downwards (southwards) on the side towards the Magellanic Clouds, which are below the plane, and upwards on the opposite side. At a distance of 12 kpc from the galactic center, for example, the maximum deviation from the principal plane is 350 to 400 pc, which is about 121 times the thickness of the gas layer. A phenomenon of this type suggests a gravitational tide, but the effect is quantitatively much too large for a simple gravitational explanation. There may be an analogy with cases reported by Zwicky and others, in which there appear to be unexpectedly large interactions between galaxies. Kerr, F. J. 1956, Proc. Radio Astr. Symposium, Sydney, Sept. 4-5 (in press). van de Hulst, H. C., Muller, C. A. and Gort J. 1954, B. A. N. 12, 117. C. S. L R.O. Radio physics Laboratory, Sydney, Australia.