The Ultra-Compact H II Region Population of the Galaxy

This thesis focuses on one of the key phases of massive star formation - ultra-compact H II regions (UCHIIs). It is now possible to build up on the existing census of these sources by exploring the dense photo-ionised gas in unprecedented resolution and at higher frequency than previously available. Such data are delivered by the CORNISH survey. A catalogue of 239 ultra-compact HII regions found in the CORNISH survey at 5 GHz and 1.5" resolution in the region 10° < l < 65°, |b| < 1° is presented. This is the largest complete and well-selected sample of UCHIIs to date and provides the opportunity to explore the global and individual properties of this key state in massive star formation at multiple wavelengths. The nature of the candidates was validated, based on observational properties and calculated spectral indices, and the analysis is presented in this work. The physical sizes, luminosities, and other physical properties were computed by utilising literature distances or calculating the distances whenever a value was not available. The near- and mid-infrared extended source fluxes were measured and the extinctions towards the UCHIIs were computed. The new results were combined with available data at longer wavelengths and the spectral energy distributions (SEDs) were reconstructed for 177 UCHIIs. The bolometric luminosities obtained from SED fitting are presented. By comparing the radio flux densities to previous observational epochs, it was found that about 5% of the sources appear to be time variable. Follow-up higher-resolution observations of unresolved CORNISH sources increased the number of UCHIIs with morphological information in the sample to 213. The UCHII region morphologies were classified with a new, multi-wavelength morphological classification scheme. Cometary UCHIIs make up 71% of the sample. This fraction is significantly higher than those found in previous, radio-based classifications. As many as 10-15% of the CORNISH UCHII regions have point-like or extended hard X-ray emission counterparts. This first high-resolution area survey of the Galactic plane shows that the total number of UCHIIs in the Galaxy is ∼ 750 - a factor of 3-4 fewer than found in previous large area radio surveys. It will form the basis for future tests of models of massive star formation.