The Dense Warm Ionized Medium in the Inner Galaxy

Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for awidely distributed highly ionized warm interstellar gas with density intermediate between the warm ionized medium (WIM) andcompact Hii regions suggests that there is a major gap in our understanding of the interstellar gas.Aims. To investigate the properties of the dense warm ionized medium in the Milky Way using spectrally resolved SOFIA GREAT[N ii] 205 m fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrallyunresolved Herschel PACS [N ii] 122m data, and spectrally resolved 12CO.Methods.We observed eight lines of sight LOS in the 20 < l < 30 region in the Galactic plane.We analyzed spectrally resolved linesof [N ii] at 205 m and RRL observations, along with the spectrally unresolved Herschel PACS 122 m emission, using excitation andradiative transfer models to determine the physical parameters of the dense warm ionized medium.We derive the kinetic temperature,and thermal and turbulent velocity dispersions from the [N ii] and RRL linewidths.Results. The regions with [N ii] 205 m emission are characterized by electron densities, n(e) 10 to 35 cm􀀀3, temperatures in therange 3400 to 8500 K, and nitrogen column densities, N(N+) 71016 to 31017 cm􀀀2. The ionized hydrogen column densitiesrange from 61020 to 1.71021 cm􀀀2 and the fractional nitrogen ion abundance, x(N+) 1.110􀀀4 to 3.010􀀀4, implying an enhancednitrogen abundance at a distance 4.3 kpc from the Galactic Center. The [N ii] 205 m emission lines coincide with CO emission,although often with an oset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions,which themselves are associated with molecular gas.Conclusions. These dense ionized regions are found to contribute & 50% of the observed [C ii] intensity along these lines of sight.The kinetic temperatures we derive are too low to explain the presence of N+ resulting from electron collisional ionization and/orproton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation fromnearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium.