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Abstract Observational studies of Hiiregion–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 Hiiregion, offering a new means of probing such interactions. We explore this hypothesis using dedicated Jansky Very Large Array observations of three Galactic Hiiregions, 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 Hiiregions—the first time this has been demonstrated for resolved sources—and evidence from¹³CO(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 Hiiregion–molecular cloud interactions. 

Abstract Diffuse ionized gas pervades the disk of the Milky Way. We detect extremely faint emission from this Galactic warm ionized medium (WIM) using the Green Bank Telescope to make radio recombination line (RRL) observations toward two Milky Way sight lines: G20, (ℓ,b) = (20°, 0°), and G45, (ℓ,b) = (45°, 0°). We stack 18 consecutive Hnαtransitions between 4.3 and 7.1 GHz to derive 〈Hnα〉 spectra that are sensitive to RRL emission from plasmas with emission measures EM ≳ 10 cm⁻⁶pc. Each sight line has two Gaussian-shaped spectral components with emission measures that range between ∼100 and ∼300 cm⁻⁶pc. Because there is no detectable RRL emission at negative LSR velocities, the emitting plasma must be located interior to the solar orbit. The G20 and G45 emission measures imply rms densities of 0.15 and 0.18 cm⁻³, respectively, if these sight lines are filled with homogeneous plasma. The observed 〈Hnβ〉/〈Hnα〉 line ratios are consistent with LTE excitation for the strongest components. The high-velocity component of G20 has a narrow line width, 13.5 km s⁻¹, that sets an upper limit of ≲4000 K for the plasma electron temperature. This is inconsistent with the ansatz of a canonically pervasive, low-density, ∼10,000 K WIM plasma. 

Abstract The cold neutral medium (CNM) is where neutral atomic hydrogen (Hi) is converted into molecular clouds, so the structure and kinematics of the CNM are key drivers of galaxy evolution. Here we provide new constraints on the vertical distribution of the CNM using the recently developedkinematic_scaleheightsoftware package and a large catalog of sensitive Hiabsorption observations. We estimate the thickness of the CNM in the solar neighborhood to beσ_z∼ 50–90 pc, assuming a Gaussian vertical distribution. This is a factor of ∼2 smaller than typically assumed, indicating that the thickness of the CNM in the solar neighborhood is similar to that found in the inner Galaxy, consistent with recent simulation results. If we consider only structures with Hioptical depthsτ> 0.1 or column densitiesN(Hi) > 10^19.5cm⁻², which recent work suggests are thresholds for molecule formation, we findσ_z∼ 50 pc. Meanwhile, for structures withτ< 0.1 or column densitiesN(Hi) < 10^19.5cm⁻², we findσ_z∼ 120 pc. These thicknesses are similar to those derived for the thin- and thick-disk molecular cloud populations traced by CO emission, possibly suggesting that cold Hiand CO are well mixed. Approximately 20% of CNM structures are identified as outliers, with kinematics that are not well explained by Galactic rotation. We show that some of these CNM structures—perhaps representing intermediate-velocity clouds—are associated with the Local Bubble wall. We compare our results to recent observations and simulations, and we discuss their implications for the multiphase structure of the Milky Way’s interstellar medium. 

Abstract The vertical distribution of cold neutral hydrogen (Hi) clouds is a constraint on models of the structure, dynamics, and hydrostatic balance of the interstellar medium. In 1978, Crovisier pioneered a method to infer the vertical distribution of Hiabsorbing clouds in the solar neighborhood. Using data from the Nançay 21 cm absorption survey, Crovisier determined the mean vertical displacement of cold Hiclouds, 〈∣z∣〉. We revisit that author’s analysis and explore the consequences of truncating the Hiabsorption sample in Galactic latitude. For any nonzero latitude limit, we find that the quantity inferred by Crovisier is not the mean vertical displacement but rather a ratio involving higher moments of the vertical distribution. The resultant distribution scale heights are thus ∼1.5 to ∼3 times smaller than previously determined. In light of this discovery, we develop a Bayesian Monte Carlo Markov Chain method to infer the vertical distribution of Hiabsorbing clouds. We fit our model to the original Nançay data and find a vertical distribution moment ratio 〈∣z∣³〉/〈∣z∣²〉 = 97 ± 15 pc, which corresponds to a Gaussian scale heightσ_z= 61 ± 9 pc, an exponential scale heightλ_z= 32 ± 5 pc, and a rectangular half-widthW_z,1/2= 129 ± 20 pc. Consistent with recent simulations, the vertical scale height of cold Hiclouds appears to remain constant between the inner Galaxy and the Galactocentric distance of the solar neighborhood. Local fluctuations might explain the large-scale height observed at the same Galactocentric distance on the far side of the Galaxy. 

Abstract The ideal spectral averaging method depends on one’s science goals and the available information about one’s data. Including low-quality data in the average can decrease the signal-to-noise ratio (S/N), which may necessitate an optimization method or a consideration of different weighting schemes. Here, we explore a variety of spectral averaging methods. We investigate the use of three weighting schemes during averaging: weighting by the signal divided by the variance (“intensity-noise weighting”), weighting by the inverse of the variance (“noise weighting”), and uniform weighting. Whereas for intensity-noise weighting the S/N is maximized when all spectra are averaged, for noise and uniform weighting we find that averaging the 35%–45% of spectra with the highest S/N results in the highest S/N average spectrum. With this intensity cutoff, the average spectrum with noise or uniform weighting has ∼95% of the intensity of the spectrum created from intensity-noise weighting. We apply our spectral averaging methods to GBT Diffuse Ionized Gas hydrogen radio recombination line data to determine the ionic abundance ratio,y⁺, and discuss future applications of the methodology. 

Abstract The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4 to 8 GHz. The nominal survey zone is 32.°3 >ℓ> −5°, ∣b∣ < 0.°5. Here, we analyze GDIGS Hnαionized gas emission toward discrete sources. Using GDIGS data, we identify the velocity of 35 Hiiregions that have multiple detected RRL velocity components. We identify and characterize RRL emission from 88 Hiiregions that previously lacked measured ionized gas velocities. We also identify and characterize RRL emission from eight locations that appear to be previously unidentified Hiiregions and 30 locations of RRL emission that do not appear to be Hiiregions based on their lack of mid-infrared emission. This latter group may be a compact component of the Galactic Diffuse Ionized Gas. There are an additional 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane. We compare these objects’ RRL data to¹³CO, Hi,and mid-infrared data, and find that these sources do not have the expected 24μm emission characteristic of Hiiregions. Based on this comparison we do not think these objects are Hiiregions, but we are unable to classify them as a known type of object. 

Context.Central bars and spirals are known to impact significantly the evolution of their host galaxies, both in terms of dynamics and star formation. Their typically different pattern speeds cause them to regularly overlap, which induces fluctuations in bar parameters. Aims.In this paper, we analyze both numerical simulations of disk galaxies and observational data to study the effect of bar-spiral physical overlap on stellar radial migration and star formation in the bar vicinity, as a function of time and galactic azimuth. Methods.We studied three different numerical models, two of which are in a cosmological context, alongside APOGEE DR17 data and the WISE catalog of Galactic HII regions. Results.We find that periodic boosts in stellar radial migration occur when the bar and spiral structures overlap. This mechanism causes net inward migration along the bar leading side, while stars along the bar trailing side and minor axis are shifted outward. The signature of bar-spiral-induced migration is seen between the bar inner Lindbald resonance and well outside its corotation, beyond which other drivers take over. We also find that, in agreement with simulations, APOGEE DR17 stars born at the bar vicinity (which are mostly metal rich) can migrate out to the solar radius while remaining on cold orbits. For the Milky Way, 13% of stars in the solar vicinity with an eccentricity <0.5 were born inside the bar, compared to 5–20% in the simulations. Bar-spiral reconnections also result in periodic starbursts at the bar ends with an enhancement of up to a factor of 4, depending on the strength of the spiral structure. Similarly to the migration bursts, these do not always happen simultaneously at the two sides of the bar, which hints at the importance of odd spiral modes. Data from the WISE catalog suggest this phenomenon is also relevant in our own Galaxy.