H I aperture synthesis observations towards galactic H II regions

Aperture synthesis observations of 21-cm H I emission and absorption towards the H II region/molecular cloud complex W 58 are presented, with angular resolution 2.5 x 4.6 arcmin (RA x Dec) and velocity resolution 2 km s-1. A large cloud, visible in emission, is found centred just north of K 3-50, and corresponds to the cloud found by Bridle and Kesteven at -26 km s-1. The velocity structure of the cloud is very complicated; it is not consistent with simple rotation but is suggestive of large-scale expanding shells or sheets of gas, moving at relative velocities approximately 10 to 20 km s-1. Such features are also indicated by the appearance of the cloud at high resolution. H I absorption towards the compact H II regions is consistent with the production of atomic hydrogen close to the H II regions themselves by photo-dissociation of H2. An absorption feature is found at -46 km s-1 towards W 58 G but not towards the other components, indicating that W 58 G lies at a much greater distance and is therefore unrelated to the rest of W 58. (author)