Hydroxyl and methylidyne radicals as tracers of dark molecular gas in a high-latitude cloud

<p dir="ltr">We observed 46 positions in a high-latitude Galactic cloud, which is the closest known molecular cloud to the Southern Galactic Pole, with the Murriyang-Parkes radio telescope. We analyse the four 18 cm OH ground-rotational state hyperfine transitions, three 9 cm CH ground-rotational state hyperfine transitions, and the Hi spin-flip transition at 1.4 GHz, and use the hydride (OH and CH) lines to measure CO-dark molecular hydrogen in this diffuse cloud. This is the first study of weak OH and CH radio spectral lines calibrated with the newly introduced software INSPECTA. Since our prioritywas to maximise the signal-to-noise (S/N) ratio for our weak lines, we used a modified calibration approach as opposed to the traditional position switching method which is generally used in spectral line radio astronomy. We find that our modified calibration approach gives objectively better results than position switching based calibration, specifically for the case of our weak and narrow spectral lines. We present the spectra for three OH and three CH transitions across the 46 positions, and have detected the 1665 MHz, and 1667 MHz lines in OH and the CH 3335 MHz and 3349 MHz lines at multiple positions. However, we failed to detect the satellite transitions at 1612 MHz, 1720 MHz and 3263 MHz at a statistically significant level in our calibrated spectra. We estimate the OH and CH column densities using the findings of previous research with judicious assumptions about the excitation temperatures of the 1667 MHz OH and 3335 MHz CH transitions. We further estimate molecular hydrogen column density from these estimates and compare it with ¹²CO(J=1–0) integrated intensity from archival Galactic CO survey data. We find that nearly 80 percent of the measured portion of the cloud is CO-dark. We consequently estimate the total molecular mass of the whole cloud and find that it is nearly 1300M_⊙, with its dark molecular gas (DMG) component amounting to 900M_⊙. Future work may include combining these results with a comprehensive analysis of the Hi data to explore the evolutionary state and conditions for molecule formation, and understanding the role of the cloud’s location, with respect to the Local Bubble, in its formation.</p>