First detection of [NII] 205 μm absorption in interstellar gas. Herschel-HIFI observations towards W31C, W49N, W51, and G34.3+0.1.
Journal article, 2014

We present high resolution [N II] 205 μm (3^P_1 − 3^P_0) spectra obtained with Herschel-HIFI towards a small sample of far-infrared bright star forming regions in the Galactic plane: 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 W 31C and W 49N over a wide range of velocities. We attribute this absorption to the warm ionised medium (WIM) and find N(N+) ≈ 1.5×10^17 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.

atomic processes

ISM: atoms

line formation

ISM: structure

Galaxy: general

ISM: abundances


Carina Persson

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

M. Gerin

Ecole Normale Superieure (ENS)

Bhaswati Mookerjea

Tata Institute of Fundamental Research

John H Black

Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics

Michael Olberg

Chalmers, Earth and Space Sciences, Onsala Space Observatory

J.R. Goicoechea

CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)

G.E. Hassel

Siena College

E. Falgarone

Ecole Normale Superieure (ENS)

F. Levrier

Ecole Normale Superieure (ENS)

K.M. Menten

Max Planck Institute

J. Pety

Institut de RadioAstronomie Millimetrique (IRAM)

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 568 37- A37

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences



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