Gaussian analysis of interstellar neutral hydrogen emission profiles has revealed a pervasive broad component with a width on the order of 34?km?s–1. When present, this component can most readily be identified in high galactic latitude directions where the H I profiles are either intrinsically weak or simple. Examination of published data reveals that this characteristic line width has been found in a variety of other H I features including compact high-velocity clouds, very-high-velocity clouds, and the Magellanic Stream. When its presence is accounted for in the analysis of H I profiles, other families of line widths at 14 and 6?km?s–1 are clearly revealed. Possible mechanisms for producing this broad background component are discussed, including temperature, turbulence, and the critical ionization velocity effect. A line width on the order of 34?km?s–1 would imply a kinetic temperature of 24,000 K, too high to keep the gas neutral; hence it should not be observed in H I emission spectra. Turbulent motions could explain a pervasive broad component, but not why it always has the same numerical value in various classes of H I emission line features. The critical ionization velocity effect hypothesis is intriguing because 34?km?s–1 is the value for helium. Clearly, this could be a coincidence but the other prominent distribution peaks correspond to two families of critical ionization velocities of abundant interstellar elements including C, N, and O (about 14?km?s–1) and metals (about 6?km?s–1). Unfortunately, the mechanism by which this effect operates, even in laboratory situations, is not clearly understood. It is suggested that further investigation of the distribution of H I component line widths by allowing for the existence of a pervasive broad underlying component may cast a clearer light on this intriguing phenomenon.
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