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

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.