H I shells, supershells, shell-like objects, and 'worms'

Abstract

The present photographic representations of the combinations of two H I surveys and high contrast photographs for particular velocities (made in order to exhibit weak H I features) have been used to prepare a list of H I shells, supershells, and shell-like objects. Attention is given to the structure of three shell-like objects associated with high velocity gas, as well as with gas at all velocities that is associated with radio continuum loops I, II, and III. The gas filaments or 'worms' found by spatial filtering to be crawling away from the galactic plane in the inner Galaxy are probably parts of shells that are open at the top. An assessment of observational data on shells and supershells indicates no unique relationship between shells and any other type of astronomical object. Stellar winds and supernovas in stellar associations are inadequate energy supplies for the largest observed supershells, unless star formation activity is unusually great. An alternative energy source, high velocity cloud collisions with the galactic disk, is consistent with several aspects of the data.