GNOMES II: Analysis of the Galactic diffuse molecular ISM in all four ground state hydroxyl transitions using Amoeba

We present observations of the four 2 Pi 3/2 J = 3/2 ground-rotational state transitions of the hydroxyl molecule (OH) along 107 lines of sight both in and out of the Galactic plane: 92 sets of observations from the Arecibo telescope and 15 sets of observations from the Australia Telescope Compact Array (ATCA). Our Arecibo observations included off-source pointings, allowing us to measure excitation temperature (Tex) and optical depth, while our ATCA observations give optical depth only. We perform Gaussian decomposition using the Automated Molecular Excitation Bayesian line-fitting Algorithm 'AMOEBA' (Petzler, Dawson, and Wardle 2021) fitting all four transitions simultaneously with shared centroid velocity and width. We identify 109 features across 38 sightlines (including 58 detections along 27 sightlines with excitation temperature measurements). While the main lines at 1665 and 1667 MHz tend to have similar excitation temperatures (median Tex(main) difference = 0.6 K, 84% show Tex(main) difference < 2 K), large differences in the 1612 and 1720 MHz satellite line excitation temperatures show that the gas is generally not in LTE. For a selection of sightlines we compare our OH features to associated (on-sky and in velocity) HI cold gas components (CNM) identified by Nguyen et al. (2019) and find no strong correlations. We speculate that this may indicate an effective decoupling of the molecular gas from the CNM once it accumulates.Comment: Accepted for publication to PASA. 41 pages, 27 figure

Probing the galactic ISM in OH absorption

Empirical thesis.Bibliography: pages 59-66.1. Introduction -- 2. Observations and data reduction -- 3. Gaussian decomposition -- 4. Results and discussion -- 5. Conclusions -- Appendices -- References.We examine the four 2π3/2, J = 3/2 ground state transitions of the hydroxyl radical (OH) along 15 sightlines through the Milky Way disk towards bright background continuum sources. We find that the OH gas along these sightlines is optically thin, consistent with the findings from the Southern Parkes Large Area Survey in Hydroxyl (SPLASH) pilot region]. We present a partially automated method for Gaussian decomposition of our spectra which identified 55 components across 28 OH-containing clouds. Three of these OH clouds had no associated 12CO emission (CO data from NANTEN).We test two methods of finding excitation temperature: a modified ‘on-off’ method, and a method where the excitation temperatures and column densities are solved for numerically .Neither method constrained Tex well. Column densities were calculated based on a range of possibleexcitation temperatures (Tex (1667) = 5-15 K), and conversions to N(H2) were made. N(OH) values were found to range from 1 - 40 x 1014cm-2, corresponding to N(H2) values ranging from1 - 40 x 1021 cm-2. We discuss our results in the context of CO-dark H2, and outline future directions for our work.Mode of access: World wide web1 online resource (66 pages) colour illustration