[C II] 158 <i>μ</i>m self-absorption and optical depth effects
C. Guevara, J. Stutzki, V. Ossenkopf-Okada, R. Simon, J. P. Pérez-Beaupuits, H. Beuther, S. Bihr, R. Higgins, U. Graf, R. Güsten
Abstract
Context. The [C II ] 158 μ m far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [C II ] emission. Aims. Our aim is to determine whether the complex profiles observed in [ 12 C II ] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [ 12 C II ] and isotopic [ 13 C II ] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [ 13 C II ] emission lines, along with the [ 12 C II ] emission lines, with a high signal-to-noise ratio. We first derived the [ 12 C II ] optical depth and the [C II ] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [C II ] gas: column density and excitation temperature. Results. We find moderate to high [ 12 C II ] optical depths in all four sources and self-absorption of [ 12 C II ] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent A v of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [C II ] gas, with an equivalent A v ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [C II ] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins.