Tracing Gas Kinematics and Interactions between H <scp>II</scp> Regions and Molecular Clouds Using VLA Observations of Recombination Lines and Hydroxyl

Abstract Observational studies of H ii 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 H ii region, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array observations of three Galactic H ii 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 toward two of our sources, G049.205−0.343 and G034.256+0.145, finding agreement between key predictions of the flip hypothesis and the observed multiwavelength spectra, kinematics, and morphology. Specifically, we demonstrate a strong spatial and kinematic association between the OH flip and the ionized gas of the H ii 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 nondisturbed 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 H ii region–molecular cloud interactions.