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1. Milky Way Structure Revealed by Galactic HII Regions

   Wenger, Trey; Terrey, Catie (January 2023, Bulletin of the American Astronomical Society)

   Despite decades of effort, the morphological structure of the Milky Way remains hidden behind dust extinction, small number statistics, and complicated datasets. HII regions, the volumes of ionized gas surrounding recently-formed massive stars, are a classic tracer of spiral arms in galaxies. Over the past decade, the HII Region Discovery Surveys have nearly tripled the number of known Galactic HII regions. With the new Galaxy-wide flux-limited sample of Milky Way HII regions, we are poised to revolutionize our understanding of spiral structure across the Galactic disk. Traditional methods of fitting Galactic structure models to the three-dimensional positions of these nebulae are impossible, however, since most Galactic HII regions lack accurate distance determinations. We are developing a novel machine learning approach that uses simulation based inference to fit complex models of Galactic structure to the complicated position-position-velocity HII region dataset, thereby removing the need for accurate distances. Using simulated observations, we demonstrate the efficacy of this new technique and its potential to reveal the structure of spiral arms across the Milky Way. 

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2. The Impact of HII Regions on the Interstellar Medium of our Galaxy

   https://doi.org/10.33915/etd.3921  

   Luisi, M.; Anderson, L. D.; Liu, B.; Balser, D. S.; Bania, T. M.; Wenger, T. V.; Haffner, L. (June 2020, American Astronomical Society Meeting Abstracts #236)

   The interstellar medium (ISM) of galaxies like the Milky Way contains low-density diffuse ionized gas (DIG). High-mass stars emit large amounts of ionizing radiation and it is believed that a fraction of this radiation escapes from their HII regions and into the ISM where it is responsible for maintaining the ionization of the DIG. The goal of this dissertation work is to better understand how the radiation produced by high-mass stars is able to leak from the HII regions, how the radiation field changes during this process, and how the radiation affects the ambient ISM. Using Green Bank Telescope (GBT) pointed radio recombination line (RRL) data of a subset of Galactic HII regions and fully-sampled RRL maps from the GBT Diffuse Ionized Gas Survey (GDIGS), we show that the morphology of the photodissociation region surrounding an HII region strongly affects the amount of leaking radiation. We also show that physically large HII regions affect the surrounding ISM out to larger distances from the region. This indicates that giant HII region complexes may have a greater effect on maintaining the ionization of the DIG. We find a correlation between dust temperature and integrated RRL intensity, suggesting that the same radiation field that heats the dust also maintains the ionization of the DIG. 

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3. A VLA Census of the Galactic HII Region Population

   Armentrout, W.; Anderson, L.; Wenger, T.; Balser, D.; Bania, T. (January 2020, American Astronomical Society meeting #235)

   The Milky Way contains a significant number of unconfirmed HII regions, the archetypical tracers of Galactic high-mass star formation. There are over 2000 confirmed HII regions in the Milky Way, but our Milky Way surveys are deficient by several thousand HII regions when compared to external galaxies with similar star formation rates. This is odd given our close proximity to these Milky Way HII regions compared to distant extragalactic sources. Through sensitive 9 GHz radio continuum observations with the Jansky Very Large Array, we explore a faint class of unconfirmed HII region candidates to put limits on the total population of Galactic HII regions. We show that stars of spectral type B2 create HII regions with similar infrared and radio continuum morphologies as those HII regions created by O-stars. We achieve this by measuring the peak and integrated radio flux densities from these faint infrared-identified objects and comparing the inferred Lyman continuum fluxes with spectral models of OB-stars. From our 50% detection rate of previously "radio quiet" sources from the WISE Catalog of Galactic HII regions, we expect a lower limit of ~7000 HII regions in our Galaxy. We have not yet discovered the vast majority of the Milky Way's HII regions. 

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4. Azimuthal Metallicity Structure in the Milky Way Disk Revealed by Galactic HII Regions

   Wenger, T.; Balser, D. S.; Anderson, L. D.; Bania, T. M. (January 2020, American Astronomical Society meeting #235)

   The present-day metallicity structure of the Galactic disk is the product of billions of years of chemodynamical evolution. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to measure 8-10 GHz radio continuum and hydrogen radio recombination line (RRL) emission toward 82 Galactic HII regions. Since collisionally excited lines from metals (e.g., oxygen, nitrogen) are the primary cooling mechanism in ionized gas, the HII region electron temperature is empirically correlated to the nebular metallicity. We use the RRL-to-continuum ratio to derive electron temperatures and infer metallicities of these Galactic HII regions. Including previous single dish studies, there are now 167 nebulae with radio-determined electron temperatures and either parallax or kinematic distance determinations. The HII region oxygen abundance gradient across the Milky Way disk has a slope of -0.052 ± 0.004 dex/kpc. We find significant azimuthal structure in the metallicity distribution. The slope of the oxygen abundance gradient varies by a factor of ~2 between Galactocentric azimuths of 30 degrees and 100 degrees. Such azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms. 

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5. Can Formaldehyde Absorption Measurements Resolve the Kinematic Distance Ambiguity?

   Luisi, Matteo; Anderson, Loren; Liu, Bin; Balser, Dana; Bania, Thomas; Wenger, Trey; Haffner, L. (June 2022, American Astronomical Society Meeting #240)

   Kinematic distance determinations are complicated by a kinematic distance ambiguity (KDA) within the Solar orbit. For an axisymmetric Galactic rotation model, two distances, a "near" and "far" distance, have the same radial velocity. Formaldehyde (H2CO) absorption measurements have been used to resolve the KDA toward Galactic HII regions. This method relies on the detection of H2CO absorption against the broadband radio continuum emission from HII regions. H2CO absorption at velocities between the HII region velocity and the maximum velocity along the line of sight (the tangent point velocity) implies that the HII region lies at the far kinematic distance whereas a lack of absorption implies that it lies at the near kinematic distance. The reliability of KDA resolutions using H2CO is unclear, however, as disagreements between distances derived using H2CO absorption and those derived using other methods are common. Here we use new H2CO and radio recombination line data from the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) to test whether H2CO absorption measurements can accurately resolve the KDA for 44 Galactic HII regions that have known distances from maser parallax measurements. For each of the 44 HII regions we determine whether the parallax distance is consistent with either the near or the far kinematic distance. We find that the Galactic distribution of H2CO is too sparse to reliably determine whether an HII region is at its near kinematic distance. The H2CO method also incorrectly resolves the KDA for 80% of HII regions that it places at the far kinematic distance; in such cases H2CO absorption may be caused by other sources of radio continuum emission (possibly the CMB, diffuse free-free, or synchrotron). Our results indicate that the H2CO method is unsuitable to resolve the KDA toward Galactic HII regions. 

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