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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. 

Hiiregion heavy-element abundances throughout the Galactic disk provide important constraints to theories of the formation and evolution of the Milky Way. In LTE, radio recombination line (RRL) emission and free–free continuum emission are accurate extinction-free tracers of the Hiiregion electron temperature. Since metals act as coolants in Hiiregions via the emission of collisionally excited lines, the electron temperature is a proxy for metallicity. Shaver et al. found a linear relationship between metallicity and electron temperature with little scatter. Here we use CLOUDY Hiiregion simulations to (1) investigate the accuracy of using RRLs to measure the electron temperature and (2) explore the metallicity–electron temperature relationship. We model 135 Hiiregions with different ionizing radiation fields, densities, and metallicities. We find that electron temperatures derived under the assumption of LTE are about 20% systematically higher owing to non-LTE effects, but overall LTE is a good assumption for centimeter-wavelength RRLs. Our CLOUDY simulations are consistent with the Shaver et al. metallicity–electron temperature relationship, but there is significant scatter since earlier spectral types or higher electron densities yield higher electron temperatures. Using RRLs to derive electron temperatures assuming LTE yields errors in the predicted metallicity as large as 10%. We derive correction factors for log(O/H) + 12 in each CLOUDY simulation. For lower metallicities the correction factor depends primarily on the spectral type of the ionizing star and ranges from 0.95 to 1.10, whereas for higher metallicities the correction factor depends on the density and is between 0.97 and 1.05. 

The three-dimensional distribution of neutral hydrogen in the Milky Way disk is a key constraint on models of Galactic spiral structure, galaxy evolution, and star formation. In particular, the vertical distributions of the different phases of hydrogen (ionized, warm neutral, cold neutral, and molecular) inform our understanding of the evolution of gas between these phases. Although the scale height of the HI emission disk has been well-characterized across the Galaxy, the vertical distribution of the cold HI component is significantly more challenging to constrain due to the sensitive absorption observations required to characterize this phase. Almost four decades ago, Crovisier (1987) pioneered a kinematic method to estimate the vertical distribution of cold HI clouds in the solar neighborhood using the latest results from the Nancay 21-cm absorption survey. This method was subsequently used in other studies to constrain the vertical distribution of neutral and molecular clouds. We have discovered an error in Crovisier's method that can lead to a factor of two inaccuracy in the inferred scale height. We will discuss the mistake and, using the original Nancay data and a corrected method based on Crovisier's technique, demonstrate the magnitude of the error in the inferred scale height of the local cold HI disk. Furthermore, we will introduce a new Monte Carlo Markov Chain method to infer the vertical distribution of HI absorbing clouds with fewer assumptions and better accuracy. This method will be used with the latest HI absorption data from the Galactic ASKAP HI survey of the Milky Way disk to provide an unprecedented view of the 3D distribution of the cold neutral medium in the solar neighborhood. 

Metallicity structure provides a critical constraint on the formation history and subsequent chemical evolution of the Milky Way. In thermal equilibrium the abundance of the coolants (O, N, and other heavy elements) in the ionized gas regulates the electron temperature, with high abundances producing low temperatures. Here we attempt to better calibrate this relationship between the plasma electron temperature, Te, and O/H by observing [OIII] (52 and 88 μm), [NIII] (57 μm), and [NII] (122 μm) toward 9 HII regions with the Herschel telescope. We derive Te in HII regions with radio recombination lines (RRLs) and use them as proxies for the nebular O/H abundances. We derive ionic abundance ratios in the well studied HII region W3A to test our calibration and analysis procedures. We find that the O/H abundance ratio varies by a factor of 5 across W3A with uncertainties that are as large as 50%, inconsistent with previous results. We suspect that the standard calibration procedures employed by Herschel, which assume the source is uniform, explains the large O/H variations in W3A. 

We present an overview of the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) and the GBT Diffuse Ionized Gas Survey at Low Frequencies (GDIGS-Low). Both GDIGS surveys trace ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission. GDIGS observes RRLs in the 4-8 GHz range and GDIGS-Low maps RRL emission at 800 MHz and 340 MHz. The nominal survey zone for both surveys is 32.3° > ℓ > -5°, |b| < 0.5°, with extensions above and below that latitude limit in select fields as well as coverage of the areas around W47 (ℓ≃37.5°), W49 (ℓ≃43°), and Cygnus X (ℓ≃80°). The goal of these surveys is to better understand the planar Diffuse Ionized Gas (DIG), including its physical properties, its dynamical state and distribution, its relationship with HII regions, and the means by which it is ionized. We discuss an analysis of the DIG around the HII region complex W43 (Luisi et. al. 2020) and a study of discrete sources of emission in the GDIGS survey area (Linville et. al. 2023). We also discuss how we will use GDIGS data to determine the ionic 4He+/ H+ abundance ratio (y+) in the DIG and how we will combine RRL observations from GDIGS and GDIGS-Low to calculate the electron density of the DIG. 

We present a comparison of the Milky Way’s star formation rate (SFR) surface density (∑_SFR) obtained with two independent state-of-the-art observational methods. The first method infers Σ_SFRfrom observations of the dust thermal emission from interstellar dust grains in far-infrared wavelengths registered in theHerschelinfrared Galactic Plane Survey (Hi-GAL). The second method determines Σ_SFRby modeling the current population of O-, B-, and A-type stars in a 6 kpc × 6 kpc area around the Sun. We find an agreement between the two methods within a factor of two for the mean SFRs and the SFR surface density profiles. Given the broad differences between the observational techniques and the independent assumptions in the methods for computing the SFRs, this agreement constitutes a significant advance in our understanding of the star formation of our Galaxy and implies that the local SFR has been roughly constant over the past 10 Myr. 

The Outer Scutum-Centaurus spiral arm (OSC) is the outermost molecular spiral arm in the Galaxy and contains the most distant known high-mass star formation regions in the Milky Way. HII regions are the archetypical tracers of high-mass star formation, and because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. We have detected HII regions at nearly 20 locations in the OSC, as far as 23.5 kpc from the Sun and 15 kpc from the Galactic center on the far side of the Galactic center. The far outer Galaxy has lower metallicity than the more inner regions of the Milky Way, with 12 + log(O/H) = 8.29 at the OSC versus 8.9 and 8.54 at the Galactic Center and the Solar neighborhood, respectively. Coupled with lower gas densities, star formation in the OSC could be similar to that of a much younger Milky Way or galaxies like the Large Magellanic Cloud. We find large reservoirs of diffuse and dense molecular gas (13CO, HCO+, HCN) in the OSC with the Argus array on the Green Bank Telescope (up to 105 Solar masses). We are also able to estimate the central ionizing sources from Very Large Array continuum observations, showing central stellar types as early as O4. Combined, these observations allow us to study chemical abundances and star formation efficiencies on the outer edge of the Milky Way, putting constraints on star formation properties towards the edge of the Galaxy's molecular disk.