First detection of the [CII] 158 micron line in the intermediate-velocity cloud Draco

TL;DR

This study reports the first detection of the 158 micron CII line in the Draco intermediate-velocity cloud, revealing shock-heated gas likely caused by cloud collisions, and highlights the role of dynamic processes in CII excitation.

Contribution

First detection of CII 158 micron emission in Draco IVC using SOFIA, linking shock heating to cloud dynamics and expanding understanding of gas excitation in low-UV environments.

Findings

CII detected at intermediate velocities in Draco

No CII detected in Spider and other low-density clouds

Shocks likely cause CII emission through cloud collisions

Abstract

High-latitude intermediate-velocity clouds (IVCs) are part of the Milky Way's HI halo and originate from either a galactic fountain process or extragalactic gas infall. They are partly molecular and can most of the time be identified in CO. Some of these regions also exhibit high-velocity cloud (HVC) gas, which is mostly atomic, and gas at local velocities (LVCs), which is partly atomic and partly molecular. We conducted a study on the IVCs Draco and Spider, both were exposed to a very weak UV field, using the receiver upGREAT on SOFIA. The 158 micron line of ionized carbon (CII) was observed, and the results are as follows: In Draco, the CII line was detected at intermediate velocities (but not at local or high velocities) in four out of five positions. No CII emission was found at any velocity in the two observed positions in Spider. To understand the excitation conditions of the gas in Draco, we analyzed complementary CO and HI data as well as dust column density and temperature maps from Herschel. The observed CII intensities suggest the presence of shocks in Draco that heat the gas and subsequently emit in the CII cooling line. These shocks are likely caused by the fast cloud's motion toward the Galactic plane that is accompanied by collisions between HI clouds. The nondetection of CII in the Spider IVC and LVC as well as in other low-density clouds at local velocities that we present in this paper (Polaris and Musca) supports the idea that highly dynamic processes are necessary for CII excitation in UV-faint low-density regions.