Tracing Massive Star Formation through a Galactic Census of Compact & Ultracompact HII Regions

Abstract

The following dissertation details the work undertaken to compile the most complete compact and ultracompact H II region catalogue to date via multi-wavelength inspection of survey data. We utilise data from the recently available SASSy 850 µm survey and explain the data reduction process leading to the final Outer Galaxy Survey. Combined with the earlier SASSy component that focuses on the Perseus arm, we were able to use the catalogue to identify massive star forming clumps in the Outer Galaxy (RGC > 8.5 kpc) and cross-match with infrared and radio data of known UC H II regions from the RMS database. We sought to compare this sample with previous existing catalogues in the Inner Galaxy to examine massive star formation as a function of galactocentric radius and particularly focusing on two distinct environmental regions (Inner and Outer Galaxy) with overall different average metallicities and gas-to-dust ratios. For this Inner Galaxy sample (RGC < 8.5 kpc), we adopted the compact H II regions from works that used similar methods to cross-match ATLASGAL with either CORNISH or RMS, depending on the location within the Galactic plane. We present a new UC H II region catalogue that more than doubles the original sample size of previous work, totalling 536 embedded H II regions and 445 host clumps. We examine the distance independent values of NLy/M and Lbol/M as proxies for massive star formation efficiency and overall star formation efficiency, respectively. We find a significant trend showing that Lbol/M decreases with increasing RGC, suggesting that the overall star formation per unit mass of the molecular clump is less in the Outer Galaxy. Further investigations with the VLA into the Outer Galaxy for 200 unmatched SASSy molecular clumps revealed seven additional 5 GHz continuum emission sources embedded within the clumps. We conclude that massive star formation is not dependent on Galactic position or environmental factors such as metallicity and the gas-to-dust ratio that are known to vary with galactocentric radius. These results will be relevant to future models attempting to use the Milky Way as a template for evolutionary and formation models of nearby galaxies where similar conditions are found but not observable to the degree of detail as seen in our own Galaxy.