2018ApJ...867..139V

A very long and nearly straight H I filament at about -60 km s^–1 in the southern Galactic hemisphere, seen nearly normal to the line of sight and well separated from low-velocity gas, has been studied in several ways in order to understand its physics, structure, and morphology. Gaussian analysis of 1800 profiles and examination of 140 declination-velocity cross sections shows that an underlying H I component, which is at least 15° long and about 1° wide, has a typical line width of 21 km s^–1. It does not appear to be in thermal pressure equilibrium with its surroundings; rather, it may be confined by a magnetic field of 18 µG. Narrow, elongated features (threads), probably unresolved in the 4' H I observations, have been identified within the boundaries of the filament. In general, each of these threads has two emission components, with line widths of the order of 8 and 3 km s^–1, which may wind around each other. Analysis suggests that these cooler components have an average density of 29 cm^–3 and may be confined by a magnetic field of 5 µG. These results, taken together, can be explained if this southern filament has magnetic substructure.