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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. 

There is relatively little known about Galactic star formation in the outer edges of the Milky Way, particularly in the Outer Scutum-Centaurus spiral arm (OSC). Lying about 15 kpc from the center of the Galaxy, the OSC was discovered in 2011 and is the most distant molecular spiral arm of the Milky Way. The OSC warps up to 4 degrees above the Galactic plane and as a result, has been excluded from the scope of many surveys of the Galactic plane, typically confined to a single degree above or below the plane. The goal of our study is to identify radio continuum from HII regions in the OSC in order to better understand the population of high-mass star formation regions in the outer Galaxy. We observed 12 HII Regions in the OSC using the Very Large Array at 10 GHz. Of our 12 targets, 7 are re-observations of undetected sources from Armentrout et al. (2017). The remaining 5 targets are sources without previously observed 10 GHz radio continuum data. We identify 10 GHz radio continuum associated with 7 of our OSC HII region targets for the first time. Assuming one dominant ionizing source per HII region, we assign spectral types from O9 to O5.5 for these sources, depending on their distance and continuum intensity. The remaining 5 nondetections represent lower-mass (B-type) star-forming regions below the sensitivity limit of our survey. These regions represent very high-mass star formation on the outer edge of the Galaxy, where densities and metallicities might be more similar to that of a much younger Milky Way or lower mass galaxies like the Magellanic Clouds. 

We determine the form of the Galactic HII region luminosity function (LF) at multiple infrared and radio frequencies. The HII region LF has been extensively studied in external galaxies, but has not received as much attention in the Milky Way. Investigations of the Galactic HII region LF have historically been limited by small sample sizes and incompleteness at lower luminosities. We find that our sample of 797 first Galactic quadrant HII regions is complete for all HII regions ionized by single O9.5 stars, and therefore provides an excellent dataset to use for extragalactic comparisons. The data are best fit by a single power law with an index of -1.73. There is little variation in the power law index with frequency. We find agreement between our result and previous studies in Hα, and therefore expect that future LF studies at wavelengths less affected by extinction should find similar results to those done in Hα. Many extragalactic LF studies suggest that a more general form of the HII region LF is a double power law. Such a form may reflect two physically distinct subpopulations; previous studies suggest the break in the double power law occurs at the transition between ionization- and density-bounded regions or regions ionized by single and multiple stars. We find that the Galactic LF is best fit by a single power law, even when it is divided into subsets by heliocentric distance, Galactocentric radius, angular size, and location relative to the spiral arms. 

HII regions are the archetypical tracers of high-mass star formation. Because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. A uniformly sensitive survey of Galactic HII regions across the disk would allow us to constrain the properties of Galactic structure and star formation. We have cataloged over 8000 HII regions and candidates in the WISE Catalog of Galactic HII Regions (astro.phys.wvu.edu/wise), but only 2000 of these are confirmed HII regions. The work is ongoing, but from our survey completeness limits and population synthesis modeling, we predict there are nearly 10,000 HII regions in the Milky Way created by a central star of type B2 or earlier. A population of especially interesting HII regions trace the Outer Scutum-Centaurus spiral arm (OSC), the most distant molecular spiral arm in the Milky Way. These regions represent star formation at low densities and low metallicities, similar to the conditions in galaxies like the Large Magellanic Cloud or a much younger Milky Way. To date, we have detected high-mass star formation at 17 locations in the OSC, with the most distant source at 23.5 kpc from the Sun and 17 kpc from the Galactic Center. They have molecular cloud masses up to 105 Msol and central stellar types as early as O4. By comparing molecular and stellar masses, we can begin to put constraints on the star formation efficiency of these distant outer Galaxy sources. We map the ionized gas using the Very Large Array at X-band in the D-configuration. We map the 13CO, HCN, and HCO+ molecular gas emission using the Argus array on the Green Bank Telescope, producing individual 5 arcmin maps with 8 arcsec resolution and 0.5 K sensitivity in 20 minutes. 

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. 

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. 

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. 

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. 

The ngVLA will create a Galaxy-wide, volume-limited sample of HII regions; solve some long standing problems in the physics of HII regions; and provide an extinction-free star formation tracer in nearby galaxies.