Intermediate-Velocity Gas in the North Galactic Hemisphere: H i Studies

This paper presents a study of the kinematics of intermediate negative velocity (∼-40 km s⁻¹ > υ > -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° 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 (|υ| < 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.