The Fractional Ionization of the Warm Neutral Interstellar Medium

TL;DR

This study uses ultraviolet spectra of nearby stars to measure ionization levels in the warm neutral interstellar medium, revealing higher ionization than expected from known sources, possibly due to past supernova activity or poorly shielded X-ray background.

Contribution

It introduces a novel method of using argon ionization as a proxy to quantify ionization in the warm neutral medium, highlighting the influence of transient astrophysical events.

Findings

Ionization level is higher than expected from known sources.

Electron density in the medium is approximately 0.04 cm^-3.

Past supernovae or poorly shielded X-ray background may explain elevated ionization.

Abstract

When the neutral interstellar medium is exposed to EUV and soft X ray radiation, the argon atoms in it are far more susceptible to being ionized than the hydrogen atoms. We make use of this fact to determine the level of ionization in the nearby, warm, neutral medium (WNM). By analyzing FUSE observations of ultraviolet spectra of 44 hot subdwarf stars a few hundred pc away from the Sun, we can compare column densities of Ar I to those of O I, where the relative ionization of oxygen can be used as a proxy for that of hydrogen. The measured deficiency [ Ar I/O I]= 0.427+/- 0.11dex below the expectation for a fully neutral medium implies that the electron density n(e) \sim 0.04cm^- 3 if n(H)=0.5cm^- 3. This amount of ionization is considerably larger than what we expect from primary photoionizations resulting from cosmic rays, the diffuse X-ray background, and X-ray emitting sources within the medium, along with the additional ionizations caused by energetic secondary photoelectrons, Auger electrons, and photons from helium recombinations. We favor an explanation that bursts of radiation created by previous, nearby supernova remnants that have faded by now may have elevated the ionization, and the gas has not yet recombined to a quiescent level. A different alternative is that the low energy portion of the soft X ray background is poorly shielded by the H I because it is frothy and has internal pockets of very hot, X-ray emitting gases.