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Calibrating the HISA temperature: Measuring the temperature of the Riegel–Crutcher cloud

Abstract:

H i self-absorption (HISA) clouds are clumps of cold neutral hydrogen (H i) visible in front of warm background gas, which makes them ideal places to study the properties of the cold atomic component of the interstellar medium. The Riegel–Crutcher (R–C) cloud is the most striking HISA feature in the Galaxy. It is one of the closest HISA clouds to us and is located in the direction of the Galactic Centre, which provides a bright background. High-resolution interferometric measurements have revealed the filamentary structure of this cloud; however, it is difficult to accurately determine the temperature and the density of the gas without optical depth measurements. In this paper, we present new H i absorption observations with the Australia Telescope Compact Array against 46 continuum sources behind the R–C cloud to directly measure the optical depth of the cloud. We decompose the complex H i absorption spectra …