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

TL;DR

This study uses VLA observations to investigate how hydroxyl (OH) spectral signatures, specifically the OH flip, can trace gas interactions between HII regions and molecular clouds, providing new insights into feedback processes.

Contribution

It demonstrates the first spatial and kinematic association of the OH flip with HII regions, validating OH as a tracer of molecular cloud interactions in resolved sources.

Findings

OH flip detected in two sources, matching predictions

Strong spatial and kinematic association between OH flip and ionized gas

Evidence of molecular gas interaction in a third source without flip

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.