Carbon recombination lines towards the Riegel-Crutcher cloud and other cold HI regions in the inner galaxy

Abstract

In the first paper of the series, Roshi, Kantharia & Anantharamaiah published the Galactic plane survey of carbon recombination lines (CRRLs) at 327 MHz. CRRLs were extensively detected from the inner Galaxy (longitudes <20◦). We report here, for the first time, the association of low-frequency CRRLs with HI self-absorbing clouds in the inner Galaxy and that the CRRLs from the innermost ∼10◦ 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 multifrequency CRRLs 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 μm line whereas the dominant heating process in the cloud interior is photoelectric emission. Constraints on the far-ultraviolet flux (G0 ∼ 4–7) falling on the R–C cloud are obtained by assuming thermal balance between the dominant heating and cooling processes. The H2 formation rate per unit volume in the cloud interior is ∼10−10– 10−12 s−1 cm−3, which far exceeds the H2 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 is ubiquitous in the inner Galaxy and forms an important part of the interstellar medium. Our analysis shows that combining CRRLs and HI data can give important insights into the nature of this cold gas. We also estimate the integration times required to image the CRRL-forming region with the upcoming Square Kilometre Array Pathfinders. Imaging with the Murchison Widefield Array telescope is feasible with reasonable observing times.