The fine-structure of the next inner spiral arm.

Summary. The spiral arms of the Galaxy are not just a phenomenon of the galactic plane. Because of the inaccuracy of current distance determinations of spiral tracers, any fine structure of the spiral arms has to be investigated by an analysis of the positions on the celestial sphere. Such fine structure perpendicular to the galactic plane is revealed by wide-angle photographs (Fig. I) and photoelectric surface photometry (Fig. 2) of the Southern Milky Way in the ultraviolet. The observed distribution of surface-brightness is closely related to the distribution of open clusters and OB-associations (Figs. 1 and 3). Furthermore, it is restricted to the region 1=2700... 200 while the remaining galactic longitudes show no features as prominent as those discussed. Three main filaments with an average length of around 400 and an inclination to the galactic equator of about 120 are seen. The pattern may thus be described as "shingle-like" - The structure in the next-inner spiral arm - I may heavily be influenced, or even simulated by local interstellar dust within a few hundred pc. Therefore, to ascertain the reality of the shingle-like structures as fine-structure of arm - I, one has to consider the possible effects of the distribution of local dark clouds. The large-scale distribution of dark clouds (Fig. 4) has no similarity with the distributions of open clusters or surface-brightness. The small-scale distribution of dark clouds (Fig. 5) shows no clear-cut correlation or anti- correlation to the shingles, it is however, modulating the structure to some extent. The interstellar reddening inside the shingles is rather larger than outside (Fig. 6), although the number of stars with known reddening is comparatively small. On the other hand, the discrete radio sources which are observed quite independently of interstellar absorption, show a general correlation to the filamentary pattern (Fig. 7). Thus, there is strong evidence to assume a fine structure of spiral arm - I. Its projection on the celestial sphere results in the observed distribution of ultraviolet surface-brightness and young objects. The shingles are then at a distance of 1.5 kpc with an average length of 1200 pc and a thickness of less than 70 pc. A similar shingle4ike pattern shows up in the radio- continuum isophotes at 11 cm wavelength between 1=3240 and 3340 (Fig. 8). Apparently, here we are looking perpendicular to spiral arm - II at a distance of 4.2 kpc. The local spiral arm presents a similar structure inclined about 100 to the galactic plane and having a thickness of 60 pc (Fig. 9). Photographs of extragalactic systems (Fig. 10) of similar type as our Galaxy frequently show broken-up spiral-arms. Average length, thickness and inclination of the fragments resemble the fine-structure found in our own Galaxy. Key words: galactic structure - spiral arms - dark clouds