[C II] 158 μm line detection of the warm ionized medium in the Scutum-Crux spiral arm tangency

Context. The Herschel HIFI GOT C+ Galactic plane [C II] spectral survey has detected strong emission at the spiral arm tangencies. Aims. We use the unique viewing geometry of the Scutum-Crux (S-C) tangency near l = 30 degrees to detect the warm ionized medium (WIM) component traced by [C II] and to study the effects of spiral density waves on Interstellar Medium (ISM) gas. Methods. We compare [C II] velocity features with ancillary H I, (CO)-C-12 and (CO)-C-13 data near tangent velocities at each longitude to separate the cold neutral medium and the warm neutral + ionized components in the S-C tangency, then we identify [C II] emission at the highest velocities without any contribution from (CO)-C-12 clouds, as WIM. Results. We present the GOT C+ results for the S-C tangency. We interpret the diffuse and extended excess [C II] emission at and above the tangent velocities as arising in the electron-dominated warm ionized gas in the WIM. We derive an electron density in the range of 0.2-0.9 cm(-3) at each longitude, a factor of several higher than the average value from H alpha and pulsar dispersion. Conclusions. We interpret the excess [C II] in S-C tangency as shock compression of the WIM induced by the spiral density waves.