The Fractional Ionization of the Warm Neutral Interstellar Medium

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 ± 0.11 dex below the expectation for a fully neutral medium implies that the electron density n(e) ≈ 0.04 cm −3 if n(H) = 0.5 cm −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.

The ESO UVES Advanced Data Products Quasar Sample – IV. On the deficiency of argon in DLA systems

“…• The average [Ar/α] abundance in DLAs is ≃ 0.2 dex lower than that found in warm interstellar regions of the Milky Way Jenkins (2013). The fact that the argon deficiency is stronger in DLAs than in the Galactic ISM is consistent with the strong rise of the metagalactic ionising background known to take place from z = 0 to z ≈ 3, the typical redshift of our sample.…”

“…The stronger underabundance in DLA systems is remarkable, considering that the H i column densities of the Galactic lines of sight with Ar i measurements are significanty lower 6 than the H i column densities in DLA systems. Therefore, H i shielding of ionising radiation should be less effective in the Galactic regions investigated by Jenkins (2013) than in the DLAs of our sample. As a result, for a given flux of ionising photons, argon should be more ionised in the ISM than in DLAs.…”

“…This scenario would yield a stronger Ar i ionisation with increasing metallicity, in line with the observed trend. The trend may also be reinforced by local production of ionising photons from the hot gas heated by supernovae, as suggested by Jenkins (2013) for the Galactic ISM.…”

“…solar) abundances are expected when the ionising spectrum is soft. In our own Galaxy, neutral argon is found to be deficient in low-density interstellar regions (Sofia & Jenkins 1998;Jenkins et al 2000;Lehner et al 2003;Jenkins 2013). However, owing to the saturation of Ar i lines, Galactic studies can only probe regions with relatively low H i column density, typically associated to warm interstellar gas.…”

“…A recent study of argon abundance in the Galactic ISM (Jenkins 2013) indicates an average argon deficiency [Ar i/O i] = −0.23 ± 0.11 dex, with the argon solar abundance 5 ξ(Ar) = 6.40 adopted here (Asplund et al 2009). The mean [Ar/α] that we measure in DLA systems is ≃ 0.2 dex lower than that measured in the ISM.…”