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

TL;DR

This study tests the HafnerFlipPaper OH flip hypothesis by mapping the four 18-cm OH satellite lines and multiple hydrogen radio recombination lines toward three Galactic H II regions with VLA L-band observations. A novel spatially coherent Gaussian decomposition of OH alongside stacking of H RRLs yields detailed maps of ionized gas and distinct OH components, enabling a robust spatial and kinematic association between the OH flip and H II regions. In two regions, G049 and G034, the OH flip components align with the ionized gas and with the parent cloud traced by CO, providing the first resolved confirmation of the flip’s proposed interpretation. The results demonstrate OH’s utility as a tracer of H II region–molecular cloud interactions and quantify the dynamical impact of feedback, while highlighting complexity and the need for larger samples and refined excitation modeling previously outlined by the HafnerFlipPaper framework.

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.

Figures (16)

• Figure 1: 8$\mu$m emission towards Galactic H ii regions G049.205$-$0.343 (top left), G034.256+0.145 (top right) and G024.471+0.492 (bottom middle) from GLIMPSE survey, overlaid with VLA continuum contours from this work. The contours are 1%, 5%, 10%, 20%, 40% 60% and 80% of the brightest 1--2 GHz radio continuum (T$_c$=1460 K, 2310 K, 810 K for G049.205$-$0.343, G034.256+0.145 and G024.471+0.492, respectively)
• Figure 2: Example of Gaussian Decomposition of OH optical depth spectra from a single sightline towards G049.205$-$0.343. The top panel shows the total model for each OH line (1612-MHz in purple, 1665-MHz in red, 1667-MHz in gray and 1720-MHz in gold). The following panels show the individual fitted Gaussian components for the 1612, 1665, 1667 and 1720-MHz lines. The bottom panel shows the fit residuals. See Section \ref{['sec:OHfit']} for details.
• Figure 3: Same as Figure \ref{['fig:G049MIMOSA']} for a single sightline towards G034.256+0.145. Note that the features at $\sim$52.5 km s$^{-1}$ and $\sim$56 km s$^{-1}$ are fit during the Gaussian decomposition process described in Section \ref{['sec:OHfit']}. These components were excluded as they corresponded to diffuse foreground clouds which are not related to the H ii region.
• Figure 4: Same as Figure \ref{['fig:G049MIMOSA']} for a single sightline towards G024.471+0.492 (RA: 18$^{\text{h}}$ 34$^{\text{m}}$ 08$^{\text{s}}$, Dec: -7$^{\circ}$ 18' 07").
• Figure 5: Maps of fitted $\langle\mathrm{H}n\alpha \rangle$ RRL spectra from G049.205$-$0.343. Top: Line of sight velocity V$_{\text{LSR}}$ from centroids of the Gaussian fits of the $\langle\mathrm{H}n\alpha \rangle$ spectra and associated fit error $\sigma$(V$_{\text{LSR}}$ ). Bottom: Fitted Gaussian FWHM line width $\Delta$V and associated fit error $\sigma(\Delta$V$)$. The contours are 1%, 5%, 10%, 20%, 40% 60% and 80% of the brightest radio continuum, $T_{\text{c}}$=1460 K. The synthesized beam size is represented by the filled black ellipse in the bottom right of each plot.