Die Spiralstruktur der Grossen Magellanschen Wolke

The spiral structure of the Large Magellanic Cloud has been investigated using the best spiral indicators (Table I). A clear spiral pattern emerges from the distribution of the optical Hii-regions with diametersD256 pc and the blue supergiants with (U-B)_o–0.60 (i.e., O-B8 Ia-O to Iab) (Figure 1). This structure is reflected in the distribution of supernova remnants, OB-associations, young open clusters, Wolf-Rayet-Stars and X-ray sources (Figure 3), andM-supergiants. The spiral features emanate from the 30 Doradus Hii-complex as centre and are completely unrelated to the LMC Bar. The structure around 30 Dor is especially well seen in the distribution and even the form (elongation) of the dark clouds (Figure 3). In Section 3 a new interpretation of the 21 cm-line data is attempted based on an analysis of the neutral hydrogen densityN _HI integrated over the line of sight. The ridge lines of the integrated 21 cm-line intensities delineate two main features with a half-intensity width of 0.9 kpc, roughly coinciding with the two main optical features. Apart from many differing details the basic structure of the distribution of Hi and of the optical spiral tracers is similar: (1) 30 Dor is the centre of density and starting-point of the spiral features, (2) two main complex arms I and II dominate the distribution, (3) the arms are fragmented optically (Ia-c and IIa-e) as well as in the Hi, start with the same steep pitch angles in directions displaced by about 60° (instead of the usual diametral symmetry of common two-armed spirals) and wind in the same directions. In the discussion (Section 4) it is first shown why the optical Hii-regions with small diametersD<6 pc do not show the spiral structure (Figure 2) due to selection and age effects. The LMC is classified exclusively from the appearance of the spiral tracers as Scp in the Hubble-system (the peculiarity is the asymmetry of the spiral structure) and ScIII-IVp in the DDO-system. Arguments are given to justify such a classification procedure. Similar galaxies may be MCG 4-31-14 and NGC 3664 S(B)IV-V:.The tidal action of our Galaxy is most probably unimportant for the spiral structure of the LMC, certainly so in the inner parts where the features are sharpest. Evidence is presented to support the view that the enormous supergiant Hii-complex 30 Doradus (M _{H II}3×10⁵ M ,M _tot=5×10⁶ M ) is the nucleus of the spiral LMC.The LMC as a pathological spiral helps us to understand together with the disturbed also the normal function of spiral formation. A density-wave type theory of spiral structure is hardly compatible with the observations; an ejection theory appears to be more promising.