Carbon Recombination Lines toward the Riegel-Crutcher Cloud and other Cold HI Regions in the inner Galaxy

TL;DR

This study links low frequency carbon recombination lines with HI self-absorbing clouds in the inner Galaxy, especially the Riegel-Crutcher cloud, revealing insights into the physical conditions and molecular formation processes in cold interstellar gas.

Contribution

It demonstrates the association of CRRLs with HISA clouds and uses combined observations to constrain physical properties and processes in these cold gas regions.

Findings

CRRLs are associated with HISA clouds in the inner Galaxy.

The R-C cloud's physical properties suggest ongoing H2 formation.

Cooling is dominated by CII 158 μm emission, heating by photoelectric effect.

Abstract

We report here, for the first time, the association of low frequency CRRL with \HI\ self-absorbing clouds in the inner Galaxy and that the CRRLs from the innermost $\sim 10^{\circ}$ of the Galaxy arise in the Riegel-Crutcher (R-C) cloud. The R-C cloud is amongst the most well known of \HI\ self-absorbing (HISA) regions located at a distance of about 125 pc in the Galactic centre direction. Taking the R-C cloud as an example, we demonstrate that the physical properties of the HISA can be constrained by combining multi-frequency CRRL and \HI\ observations. The derived physical properties of the HISA cloud are used to determine the cooling and heating rates. The dominant cooling process is emission of the \CII\ 158 \mum line whereas dominant heating process in the cloud interior is photoelectric emission. Constraints on the FUV flux (G0 $\sim$ 4 to 7) falling on the R-C cloud are obtained by assuming thermal balance between the dominant heating and cooling processes. The H$_2$ formation rate per unit volume in the cloud interior is $\sim$ 10$^{-10}$ -- 10$^{-12}$ s$^{-1}$ \cmthree, which far exceeds the H$_2$ dissociation rate per unit volume. We conclude that the self-absorbing cold \HI\ gas in the R-C cloud may be in the process of converting to the molecular form. The cold \HI\ gas observed as HISA features are ubiquitous in the inner Galaxy and form an important part of the ISM. Our analysis shows that combining CRRL and \HI\ data can give important insight into the nature of these cold gas. We also estimate the integration times required to image the CRRL forming region with the upcoming SKA pathfinders. Imaging with the MWA telescope is feasible with reasonable observing times.