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1. The GBT Diffuse Ionized Gas Survey (GDIGS): Discrete Sources

   Matteo Luisi, Dylan Linville ( January 2023 , Bulletin of the AAS)

   The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4–8 GHz. The nominal survey zone is 32.3° > l > -5°, |b| < 0.5°. Here, we analyze GDIGS Hnα ionized gas emission toward discrete sources with sizes comparable to the 2.065' GDIGS Hnα beam. We use GDIGS data to identify the correct velocity of 39 H II regions that have multiple RRL velocity components. We identify and characterize RRL emission from 88 H II regions that previously lacked measured ionized gas velocities. We additionally identify and characterize RRL emission from eight locations that appear to be previously-unidentified H II regions and 41 locations of RRL emission that do not appear to be H II regions based on their lack of mid-infrared emission. We identify 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane and we compare the objects’ RRL data to 13CO, H I and mid-infrared data. These sources do not have the expected 24 μm emission characteristic of H II regions. Based on this comparison we do not think these objects are H II regions, but we are unable to classify them as a known type of object. 

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2. GDIGS: Surveying Ionized Gas in the Inner Galactic Plane

   Linville, Dylan ; Anderson, Loren ; Luisi, Matteo ; Liu, Bin ; Wenger, Trey ; Balser, Dana ; Bania, Thomas ; Haffner, L ; Mascoop, Joshua ; Salas, Pedro ; et al ( February 2024 , Bulletin American Astronomical Society)

   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. 

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3. The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS)

   Rao, M. ; Wenger, T. V. ; Balser, D. S. ; Anderson, L. D. ; Armentrout, W. P. ; Bania, T. M. ; Budaiev, N. ; Haffner, L. M. ; Liu, B. ; Luisi, M. ( January 2020 , American Astronomical Society meeting #235)

   The Warm Ionized Medium (WIM) is a low density, diffuse ionized component of the Milky Way. The WIM is the last major component of the interstellar medium to be studied at high spatial and spectral resolution, and therefore many of its fundamental properties are not clear. Radiation from massive, OB-type stars, which live in the inner galaxy, is thought to escape discrete HII regions to ionize the WIM. However, the inner Galaxy has not been well studied due to extinction from dust at optical wavelengths. GDIGS is a fully-sampled Radio Recombination Line (RRL) survey of the inner Galactic Plane at C-band (4-8 GHz). RRL emission is not affected by extinction from dust, and GDIGS has sufficient spatial resolution to distinguish between HII regions and the WIM emission. Here we discuss the status of GDIGS and some preliminary results from the spectral analysis of the RRLs. 

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4. The Galactic Center Lobe as an H ii Region

   https://doi.org/10.3847/1538-4357/ad4d93  

   Anderson, L_D ; Luisi, Matteo ; Liu, B. ; Linville, Dylan_J ; Benjamin, Robert_A ; Hurley-Walker, Natasha ; McClure-Griffiths, N_M ; Zucker, Catherine ( June 2024 , The Astrophysical Journal)

   The Galactic center lobe (GCL) is an object ∼1° across that is located north of the Galactic center. In the mid-infrared (MIR) the GCL appears as two 8.0μm filaments between which there is strong 24μm and radio continuum emission. Due to its morphology and location in the sky, previous authors have argued that the GCL is located in the Galactic center region, created by outflows from star formation or by activity of the central black hole Sagittarius A*. In an associated paper, low-frequency radio emission indicates that the GCL must instead lie foreground to the Galactic center. If the GCL is foreground to the Galactic center, it is likely to be a type of object common throughout the Galactic disk; we here investigate whether its properties are similar to those of Galactic Hiiregions. We find that the GCL’s MIR morphology, MIR flux densities, dust temperatures, and radio recombination line properties as traced by the Green Bank Telescope Diffuse Ionized Gas Survey are consistent with those of known Galactic Hiiregions, although the derived electron temperature is low. We search for the ionizing source(s) of the possible Hiiregion and identify a stellar cluster candidate (Camargo #1092/Ryu & Lee #532) and a cluster of young stellar objects (the SPICY cluster G359.3+0.3) whose members have Gaia parallaxes distances of 1.7 ± 0.4 kpc. Taken together, the results of our companion paper and those shown here suggest that the GCL has properties consistent with those of an Hiiregion located ∼2 kpc from the Sun.

    

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5. A 5 GHz Continuum Map of the Inner Galactic Plane with the Green Bank Telescope

   Budaiev, N. ; Armentrout, W. P. ; Bania, T. M. ; Anderson, L. D. ; Balser, D. S. ; Haffner, L. M. ; Liu, B. ; Luisi, M. ; Rao, M. ; Wegner, T. ( January 2020 , American Astronomical Society Meeting Abstracts #235)

   Despite the fact that Warm Ionized Medium (WIM) is a major component of the Interstellar Medium our knowledge about it and its relationship with H II regions is very limited. Understanding the WIM better will give us insight into the formation of galaxies and evolution of high-mass star formation regions. Previous surveys of the WIM and H II regions had low spectral and spatial resolutions or looked at Hα, which suffers from extinction. In this project we attempt to get additional value from the GBT Diffuse Ionized Gas Survey (GDIGS), by making continuum maps using the existing data. The goal is to assess whether the GDIGS data can be used to measure the radio recombination line to continuum emission ratio for the diffuse ionized gas. 

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