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

Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
e-mail: Thangasamy.Velusamy@jpl.nasa.gov

Received: 29 March 2012
Accepted: 26 April 2012

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.