C. New Clues on the Sense of Rotation of Spiral Galaxies
Many discussions have been published in the astronomical literature concerning the sense of rotation of spiral galaxies. While some astronomers have claimed, both from theory and from observations, that all regular spirals are trailing their arms, others have postulated with equal conviction that the motion is exactly the opposite. To this controversy two remarks must be made. In the first place the problem has so far remained ill formulated because spiral nebulae are not rotating throughout with a constant angular velocity relative to any universal inertial system. Because of this inconstant angular velocity one part of a nebula may well rotate clockwise while the other moves anticlockwise with respect to an inertial system. In order to formulate our problem correctly we must therefore ask if the total angular momentum of a rotating galaxy is parallel or antiparallel to the "structural vector" which has the dimension of an angle. This structural vector is normal to the plane containing the spiral arms and defines the sense in which these arms are curving.
Our second remark is that so far only insufficient data have been gathered to decide in every case what the sense of rotation of a galaxy is. From the theoretical standpoint it would be truly amazing if the spiral arms were either always trailing or always advancing. Indeed, as a mechanical system a spiral galaxy is quite free to rotate either way and in general the moment of momentum vector might even be inclined at any angle relative to the plane of symmetry of a spiral galaxy. Also, since the angular velocity varies in general with the distance from the center of a galaxy, it may be expected that in some cases a part of the system will be found to rotate in one sense while the remaining part turns in the opposite direction, provided that all motions are referred to a universal inertia system. (1)
The study of the structural features of double galaxies has led the author to some new criteria which might be useful in the investigation of the sense of rotation of normal spirals and of barred spirals. In Fig. 7d we show the essential outlines of an elliptical and of a spiral nebula which are interconnected by a luminous band, the structure of which gives some clues as to the direction of rotation of the spiral.
Figure 7. Schematic drawing of the possible formation of an intergalactic bridge between two galaxies passing each other. The drawing d) represents the essential features discernible on a photograph obtained with the 200-inch telescope of the double galaxy located at R.A. 6h 44m 16s and Decl. +86o36'0" (1950). The total angular extension of the system is only about three minutes of arc and details of the various components can be recognized only with very large telescopes The sketches a, b and c depict a hypothetical sequence of events.
For purposes of discussion we assume that the globular nebula A in the sketch a) moves with the velocity V toward a passing close encounter with the elliptical nebula B which is rotating in a clockwise direction. Approaching and passing each other in the phases b) and c) the two galaxies will be subjected to the generation of tides and of countertides. The two tides facing each other will eventually join and form the bridge between the two galaxies shown in c). Because of the initial rotation of B this bridge must ultimately be ruptured. Obviously the frictional forces within B decrease as we go outward from the center and at a given distance they will be too small to force the remote parts of the bridge to turn with the contiguous spiral arm. The break shown in 7d will therefore occur. As a matter of fact, in our particular case even the spiral arm which remains from the original countertide broke off from the outlying parts of this countertide. The two arms, the two breaks, and the immediate distribution of the luminous matter beyond the breaks form a perfectly symmetrical pattern. Also it would seem obvious from the existence of the clean-cut breaks that most of the matter beyond them will eventually disappear in intergalactic space. The final result in the case shown is an open barred spiral with its two arms advancing. If the initial rotation of the elliptical nebula B had been counterclockwise, the final result of the encounter might have been the formation of a normal spiral rotating in the same sense, and with its arms trailing. It will be profitable to study the effects of encounters starting from all types of nebulae A and B and with various initial boundary conditions as to relative motion and initial moment of momentum. This approach promises to supply us with all of the necessary explanations for all possible forms of normal spirals and of barred spirals. It seems unlikely for the present that we shall have to resort to any processes of evolution starting from primordial masses of gases and of dust to arrive at a satisfactory theory of the structures of galaxies.
1 An inertia system is a system of reference in which NEWTON'S equation = F holds for every particle, where is the time derivative of the momentum vector and F is a real force acting on the particle. The origin of F must be entirely traceable to matter and must not contain any inertial terms which can be transformed away in their entirety by a change to another system of coordinates. Back.