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

Abstract

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 nearl = 30° to detect the warm ionized medium (WIM) component traced by [CII] 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, ¹²CO and ¹³CO 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 ¹²CO 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 Hα 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.