Hydrogen, Helium, and Magnetic Fields in Interstellar Space
Abstract
Analysis by multiple authors of a variety of interstellar neutral hydrogen features studied over many decades using data from different telescopes reveals a pervasive 34 km/s wide component. The traditional explanation, that the line width results from a kinetic temperature, would mean that T = 24,000 K, high enough to ionize the gas so it could not contribute to the 21-cm profile. Turbulent motions could explain a pervasive broad component, but not why it has the same numerical value in so many different types of HI features. Confusion due to telescope side lobes has been proposed as a possible explanation, but the broad feature persists in side-lobe-corrected survey data. The critical ionization velocity is a well-studied plasma phenomenon where atoms become ionized in the presence of a magnetic field when their kinetic energy relative to the plasma is equivalent to the ionization potential. The critical ionization velocity for helium is 34 km/s, which could account for the pervasiveness of this component. This result supports other evidence that the neutral hydrogen in the interstellar medium is tightly coupled to the galactic magnetic field (Clark et al. 2014; 2015). Strong support for this interpretation stems from the resulting abundance of interstellar helium, which can be estimated from the column density fraction of the 34 km/s component with respect to the entire emission profile. A derived value of 0.28 is within one σ of the cosmic abundance of helium.
- Publication:
-
American Astronomical Society Meeting Abstracts #234
- Pub Date:
- June 2019
- Bibcode:
- 2019AAS...23431602S