Tracing Gas Kinematics and Interactions between H II Regions and Molecular Clouds using VLA Observations of Recombination Lines and Hydroxyl

E. Cappellazzo; J. R. Dawson; Mark Wardle; Trey V. Wenger; Anita Hafner; Dana S. Balser; L. D. Anderson; Elizabeth K. Mahony; M. R. Rugel; John M. Dickey

arXiv · 2510.25198

Tracing Gas Kinematics and Interactions between H II Regions and Molecular Clouds using VLA Observations of Recombination Lines and Hydroxyl

E. Cappellazzo·J. R. Dawson·Mark Wardle·Trey V. Wenger·Anita Hafner·Dana S. Balser·L. D. Anderson·Elizabeth K. Mahony·M. R. Rugel·John M. Dickey

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Abstract

Observational studies of HII region-molecular cloud interactions constrain models of feedback and quantify its impact on the surrounding environment. A recent hypothesis proposes that a characteristic spectral signature in ground state hyperfine lines of hydroxyl (OH) -- the OH flip -- may trace gas that is dynamically interacting with an expanding HII region, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array (VLA) observations of three Galactic HII regions, G049.205$-$0.343, G034.256+0.145 and G024.471+0.492, in 1--2 GHz continuum emission, all four 18-cm ground-state OH lines, and multiple hydrogen radio recombination lines. A Gaussian decomposition of the molecular gas data reveals complex OH emission and absorption across our targets. We detect the OH flip towards two of our sources, G049.205$-$0.343 and G034.256+0.145, finding agreement between key predictions of flip hypothesis and the observed multi-wavelength spectra, kinematics and morphology. Specifically, we demonstrate a strong spatial and kinematic association between the OH flip and the ionized gas of the HII regions -- the first time this has been demonstrated for resolved sources -- and evidence from $^{13}$CO(1--0) data that the expected OH component originates from the non-disturbed gas of the parent cloud. While we detect no flip in G024.471+0.492, we do find evidence of interacting molecular gas traced by OH, providing further support for OH's ability to trace HII region-molecular cloud interactions.

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E. Cappellazzo, J. R. Dawson, Mark Wardle, Trey V. Wenger, Anita Hafner, Dana S. Balser, L. D. Anderson, Elizabeth K. Mahony, M. R. Rugel, John M. Dickey. 2025-10-29. Tracing Gas Kinematics and Interactions between H II Regions and Molecular Clouds using VLA Observations of Recombination Lines and Hydroxyl. https://arxiv.org/abs/2510.25198

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