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

Anita Petzler, J. R. Dawson, Hiep Nguyen, Carl Heiles, M. Wardle, M.-Y. Lee, Claire E. Murray, K. L. Thompson, Snežana Stanimirović

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Abstract

We present observations of the four 2Π3/2J=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, & Wardle 2021, ApJ, 923, 261) 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 |ΔTex(main)| =0.6 K, 84% show |ΔTex(main)| < 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, ApJ, 880, 141) and find no strong correlations. We speculate that this may indicate an effective decoupling of the molecular gas from the CNM once it accumulates.

Original languageEnglish
Article numbere015
Pages (from-to)1-36
Number of pages36
JournalPublications of the Astronomical Society of Australia
Volume40
DOIs
Publication statusPublished - 28 Feb 2023

Bibliographical note

Copyright The Author(s), 2023. Published by Cambridge University Press on behalf of the Astronomical Society of Australia. c The Author(s), 2023. Published by Cambridge University Press on behalf of the Astronomical Society of Australia.

Keywords

  • ISM: molecules
  • galaxies: ISM
  • radio lines: ISM

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