Projected dark cloud distribution of the Milky Way

The dark cloud catalogs of Lynds (1962) for the northern hemisphere and Feitzinger and Stuewe (1984) for the southern hemisphere are combined. By calibrating the opacity classes, it is demonstrated that the above data base is complete and uniform. For the total Milky Way band and for the northern, anticenter, and southern sectors, the obscuration percentages of the sky are deduced, as well as average cloud areas, cloud distributions in Galactic longitude and latitude, and mean spatial distances. The differences between the three sectors are discussed and interpreted. The morphological classification of dark clouds according to van den Bergh (1972) fits into the picture of the local Galactic structure. The flattening of the clouds (opacity classes 5 and 6) seems to be somewhat aligned with the Galactic plane, whereas clouds with lower opacities show elongations and alignments with respect to each other and to the galactic plane. By means of the stellar-statistical integral equation, the intrinsic dark cloud area distribution and absolute number densities are calculated. Comparisons are made with the size distribution of molecular clouds. 53 references