First detection of [N II] 205 μm absorption in interstellar gas: Herschel-HIFI observations towards W31C, W49N, W51, and G34.3+0.1.

We present high resolution [N II] 205 μm (3P1 – 3 P0) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: W31C (G10.6–0.4), W49N (G43.2–0.1), W51 (G49.5–0.4), and G34.3+0.1. All sources display an emission line profile associated directly with the H II regions themselves. For the first time we also detect absorption of the [N II] 205 μm line by extended low-density foreground material towards W31C and W49N over a wide range of velocities. We attribute this absorption to the warm ionised medium (WIM) and find N(N+) ≈ 1.5 × 1017 cm-2 towards both sources. This is in agreement with recent Herschel-HIFI observations of [C II] 158 μm, also observed in absorption in the same sight-lines, if ≈7–10% of all C+ ions exist in the WIM on average. Using an abundance ratio of [N]/[H] = 6.76 × 10-5 in the gas phase we find that the mean electron and proton volume densities are ~0.1–0.3 cm-3 assuming a WIM volume filling fraction of 0.1–0.4 with a corresponding line-of-sight filling fraction of 0.46–0.74. A low density and a high WIM filling fraction are also supported by RADEX modelling of the [N II] 205 μm absorption and emission together with visible emission lines attributed mainly to the WIM. The detection of the 205 μm line in absorption emphasises the importance of a high spectral resolution, and also offers a new tool for investigation of the WIM. [ABSTRACT FROM AUTHOR]