On the Stability of Interstellar Neutral Hydrogen Clouds at Large Z-heights

Abstract

Galactic neutral hydrogen (H i) data drawn from several large-scale surveys have been used to study 32 small clouds found between Galactic longitudes 70° and 140° and latitudes −20° and −45°. This places them below the Perseus spiral arm in the second quadrant of Galactic longitude. The velocities of the sample range from −30 to −57 km s−1, and their z-heights are between −1.5 and −3.7 kpc. The mapped clouds are localized brightness enhancements along the length of interstellar H i filaments. This H i may have been propelled into the Galactic halo by the action of many supernovae and stellar winds originating in the abundant star-forming regions in this longitude region of the Perseus spiral arm. Adopting this model allows the distance to each cloud to be derived. Their emission profiles are characterized by narrow lines ≈5 km s−1 wide. Application of the virial theorem for the ensemble of features indicates that an average magnetic field of 1.0 ± 0.4 μG can balance internal pressure.