On the Critical Ionization Velocity Effect in Interstellar Space and Possible Detection of Related Continuum Emission

Interstellar neutral hydrogen (HI) emission spectra manifest several families of linewidths whose numerical values (34, 13, and 6 km/s) appear to be related to the critical ionization velocities (CIVs) of the most abundant interstellar atomic species. Extended new analysis of HI emission profiles shows that the 34-km/s-wide component, which probably corresponds to the CIV for helium, is pervasive. The 34-km/s-wide linewidth family is found in low-velocity (local) HI profiles and in the so-called high-velocity clouds (HVCs). In addition, published studies of HI linewidths found in the Magellanic Stream, Very High Velocity Clouds, and Compact HVCs, all of which are believed to be intergalactic, have noted that the typical values are of the same order. If the CIV effect does play a role in interstellar space, it may be expected to produce locally enhanced electron densities where rapidly moving neutral gas masses interact with the surrounding plasma. Evidence that suggests that this phenomenon is occurring in interstellar space is presented. It manifests as a spatial association between peaks in HI structure offset with respect to peaks in high-frequency radio continuum data obtained with the Wilkinson Microwave Anisotropy Probe.