Intermediate-velocity gas in the northern Galactic hemisphere: H I studies

This paper presents a study of the kinematics of intermediate negative velocity (similar to -40 km s(-1) > upsilon > -100 km s(-1)) neutral hydrogen toward the north Galactic pole using data from the Bell Laboratories 21 cm sky survey. The majority of the intermediate-velocity gas is contained in three large coherent features covering nearly a third of the northern Galactic hemisphere. Contour plots of the emission and position-velocity diagrams reveal coherent velocity patterns spanning velocities from -100 km s(-1) to -20 km s(-1), and extending nearly 100 degrees across the sky. A few high-velocity cloud complexes appear to be kinematically related to the intermediate-velocity gas; a clear relationship between low-velocity (\upsilon\ < 20 km s(-1)) gas and intermediate velocity features is not apparent from the data. Distance estimates derived from absorption-line data by several. authors are correlated with the H I features to determine the masses of the intermediate-velocity gas features. The morphology, kinematics, distances, and masses are used to evaluate models for the origin of the intermediate-velocity gas, and three broad classes, infall, galactic fountains, and superbubbles, are found to be potentially viable.