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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. 

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.