Structure in the Milky Way

The structure of the Milky Way is a constraint on theories of Galactic formation and evolution. HII regions, the zones of ionized gas surrounding recently formed high-mass stars, are important tracers of structure in both the Milky Way and other galaxies. Using the most complete sample of these Galactic high-mass star forming regions (HMSFRs) to date, we investigate the morphological and chemical structures in the Milky Way disk.

Mapping Galactic structure with HMSFRs requires accurate distances to these tracers. Unfortunately, only kinematic distances are available for most of the HMSFRs in the Galaxy. We develop a novel Monte Carlo kinematic distance method and compare the kinematic distances and parallax distances of 75 Galactic HMSFRs. The Monte Carlo method gives more accurate kinematic distances than those derived using traditional methods. The median difference between the Monte Carlo kinematic distances and parallax distances is 17% (0.42 kpc). We find that, for a large portion of the Galaxy, the kinematic distance uncertainty is up to 10 times smaller than the parallax distance uncertainty.

The census of Galactic HII regions is vastly incomplete in the southern sky. The Southern HII Region Discovery Survey (SHRDS) is completing this census by identifying new HII regions from their radio recombination line (RRL) emission. We use the Australia Telescope Compact Array to measure 4-10 GHz radio continuum and hydrogen RRL emission from HII region candidate targets. Thus far, the SHRDS has discovered 295 heretofore unknown Galactic HII regions. This increases the number of known nebulae in the surveyed zone by 82% to 568. Upon the completion of the SHRDS, we will have a catalog of all Galactic HII regions ionized by at least one O-type star.

The metallicity structure of the Milky Way disk stems from the chemodynamical evolutionary history of the Galaxy. We use the National Radio Astronomy Observatory Karl G. Jansky Very Large Array to measure the RRL-to-continuum brightness ratios of 82 Galactic HII regions. We then derive the electron temperatures and metallicities for these nebulae. Since collisionally excited lines from metals (e.g., oxygen) are the dominant cooling mechanism in HII regions, the nebular metallicity can be inferred from the electron temperature. Including previous single dish studies, there are now 167 nebulae with accurate electron temperature and distance determinations. We find an oxygen abundance gradient across the Milky Way disk with a slope of +0.004 +0.052/-0.003 dex/kpc. We also find azimuthal structure in the metallicity distribution. The slope of the gradient varies by a factor of ~2 with Galactocentric azimuth. This azimuthal structure is consistent with simulations of Galactic chemodynamical evolution influenced by spiral arms.

Finally, we explore the spiral structure of the Milky Way by constructing a simple morphological model and using it to constrain the spiral structure of the Galaxy. The model posits the neutral gas, molecular gas, and HMSFR distributions and includes parametrizations of the kinematics of the Galactic disk, the morphology of the spiral arms, and the warp of the Galactic plane. We compare the modeled emission with observations in a novel way. We use a Bayesian Markov Chain Monte Carlo analysis to estimate the optimal model parameters that reproduce the observed Galactic longitude, latitude, and velocity distributions of HI 21 cm hyperfine emission, 12CO (J=1-0) emission, and the number of HII regions. This is an ongoing study. Our analysis is able to recover the correct model parameters for simulated datasets, but it fails to reproduce the observed gas and HMSFR distributions. We identify degeneracies between parameters as the likely cause and assess the future capabilities of our model.