[CII] 158 micron line detection of the warm ionized medium in the Scutum--Crux spiral arm tangency

TL;DR

This study uses [CII] emission data from the Galactic plane to detect and analyze the warm ionized medium in the Scutum-Crux spiral arm tangency, revealing higher electron densities and effects of spiral density waves.

Contribution

It provides the first detailed detection and analysis of the WIM in the S-C tangency using [CII] emission, highlighting the impact of spiral density waves on the ISM.

Findings

Detected extended [CII] emission associated with the WIM.

Derived higher electron densities (0.2-0.9 cm^-3) than previous estimates.

Identified shock compression effects from spiral density waves on the WIM.

Abstract

HIFI GOT C+ Galactic plane [CII] spectral survey has detected strong emission at the spiral arm tangencies. We use the unique viewing geometry of the Scutum-Crux (S-C) tangency near i = 30degs 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. We compare [CII] velocity features with ancillary HI, 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 [CII] emission at the highest velocities without any contribution from 12CO clouds, as WIM. We present the GOT C+ results for the S-C tangency. We interpret the diffuse and extended excess [CII] 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 Halpha and pulsar dispersion. We interpret the excess [CII] in S-C tangency as shock compression of the WIM induced by the spiral density waves.