2017A&A...607A..59L

Context. The warm ionized medium (WIM) occupies a significant fraction of the Galactic disk. Determining the WIM properties at the leading edge of spiral arms is important for understanding its dynamics and cloud formation.
Aims. We derive the properties of the WIM at the inner edge of the Scutum arm tangency, which is a unique location in which to disentangle the WIM from other components, using the ionized gas tracers C⁺ and N⁺.
Methods. We use high spectral resolution [CII] 158µm and [NII] 205µm fine structure line observations taken with the upGREAT and GREAT instruments, respectively, on SOFIA, along with auxiliary HI and ¹³CO observations. The observations consist of samples in and out of the Galactic plane along 18 lines of sight (LOS) between longitude 30° and 32°.
Results. We detect strong [NII] emission throughout the Scutum tangency. At V_LSR=110 to 125km/s where there is little, if any, ¹³CO, we are able to disentangle the [NII] and [CII] emission that arises from the WIM at the arm's inner edge. We find an average electron density ∼0.9cm^–3 in the plane, and ∼0.4cm^–3 just above the plane. The [NII] emission decreases exponentially with latitude with a scale height ∼55pc. For V_LSR<110km/s there is [NII] emission tracing highly ionized gas throughout the arm's molecular layer. This ionized gas has a high density, n(e)∼30cm^–3, and a few percent filling factor. We also find evidence for [CII] absorption by foreground gas.
Conclusions. [NII] and [CII] observations at the Scutum arm tangency reveal a highly ionized gas with average electron density about 10 to 20 times those of the interarm WIM, and is best explained by a model in which the interarm WIM is compressed as it falls into the potential well of the arm. The widespread distribution of [NII] in the molecular layers shows that high density ionized gas is distributed throughout the Scutum arm. The electron densities derived from [NII] for these molecular cloud regions are ∼30cm^–3, and probably arise in the ionized boundary layers of clouds. The [NII] detected in the molecular portion of the spiral arm arises from several cloud components with a combined total depth ∼8pc. This [NII] emission most likely arises from ionized boundary layers, probably the result of the shock compression of the WIM as it impacts the arm's neutral gas, as well as from extended HII regions.