Probing the Neutral Fraction of the Warm Ionized Medium via [NI] 5200

Abstract

Most of the ionized mass in the Milky Way is in the Warm Ionized Medium (WIM) and not in the bright H~II regions. The WIM is traced by dispersion measure and has been extensively studied in recombination lines (primarily, H$α$) and optical nebular lines (primarily, S+ and N+). The observations can be well explained by a photo-ionized nebula with a low ionization parameter. It is generally thought that the source of ionization (and heating) of the WIM is due to Lyman continuum leaking from HII regions which are concentrated in the Galactic plane. The rays of the diffuse Galactic Lyman-continuum radiation field incident on the Warm Neutral Medium (WNM) are absorbed, forming an ionized skin. In nebulae with low-ionization parameter the transition from ionized gas to neutral gas is gradual, unlike the case for HII regions with their sharp Stromgren spheres. The transition region is warm enough to excite oxygen and nitrogen atoms to emit [OI] 6300,6363 and [NI] 5198,5200. Domgorgen & Mathis (1994) recognized the value of [OI] 6300 as a diagnostic of the fraction of the diffuse continuum that is absorbed by the WNM and therefore constrains the fraction of the diffuse Lyman continuum that escapes to the halo. Unfortunately, observations of Galactic [OI] 6300 have been stymied by bright [OI] 6300 airglow emission. [NI] 5200,5198 has been a historically less popular probe because this doublet is less luminous than the oxygen doublet. However, we point out that the [NI] airglow is two orders of magnitude smaller than that of [OI]. Furthermore, even in the presence of comparable airglow, the WIM [NI] emission can be inferred using the doublet intensity ratio for which a medium-resolution spectrometer such as the Local Volume Mapper will suffice. Separately, we note, in extragalactic systems, that [OI]6300/[NI]5200 is a robust measure of the O/N abundance ratio.