The H-alpha Emission Signature: The Role of Critical Ionization Velocity in Interstellar Space (2310.09389v1)

Abstract: Gaussian decomposition of HI4PI profiles was done for two regions along high velocity Complex M, the first with corresponding H-alpha emission and the second without. In the former, we detected the Critical Ionization Velocity (CIV) signature, a line width of 25 km/s, of H-alpha. In low-velocity gas, the CIV effect ionizes the atoms and produces the signatures of He (34 km/s), CNO (13.4 km/s), and the metals (around 6 km/s). In anomalous-velocity gas, the CIV effect ionizes the ions and produces the signatures of ionized CNO (20 km/s) and-or H-alpha (25.5 km/s), as well as the CIV signatures of metals around 6 km/s. These distinct CIV signatures indicate that the physical conditions are different in each velocity regime. The particles in the low-velocity gas cloud are initially neutral, the temperatures can be cold (50-100 K), and the abundances can be cosmic. The densities need to be high enough to impose regular recombination. The anomalous-velocity gas, on the other hand, initially contains a significant population of ionized and excited particles. The temperature and abundance values may be similar to those that characterize low-density gas, but the densities must be low enough so that the plasma is essentially collision-less, similar to the conditions in the solar wind.