On the Direction of Rotation in Spiral Nebulae.

Abstract

The question as to t~he true direction of tilt of spiral nebulae which are inclined toward the line of sight is the subject of a new discussion here. A photometric study of the distribution of intensity and color in indisputable cases, like NGC 4565, compared with typical cases, like NGC 7331, 4216, 4258, leads to a conclusion regarding the true tilt of the last-mentioned objects which is identical with the one derived by one of us in earlier papers. The importance of the "secondary" absorption on the distant half of the nebula is emphasized on the basis of new evidence. The results indicate that the dissolution of a system into spiral structure proceeds in such a way that the arms open up in the direction of rotation. The projection effects of the distribution of novae, variables, and globular clusters in M 31 are discussed. A comparison between the distributions supports strongly the contention that the brighter side of the nebula is the nearer one. Kinematical considerations, supporting the opinion that the spiral arms open up in the direction of rota- tion, are discussed for M 81 and M 33. The case of NGC 2681, which shows an outer diffuse spiral struc- ture of a direction of winding opposite to that of the interior main spiral structure, illustrates conclusions drawn from dynamical theory. It is also of importance for the interpretation of NGC 3190, though this nebula is, no doubt, to be considered as a singular object on account of disturbing effects from its neigh- bor, NGC 3187. I. INTRODUCTIO