Physical and chemical characterization of Galactic HII regions

High-mass stars (> 8 M⊙) play a crucial role in shaping the dynamics and evolution of the interstellar medium (ISM) and galaxies, through their intense radiation, powerful stellar winds, shocks, and supernova explosions. Despite their significance, several fundamental questions concerning their formation, feedback, and regulation of subsequent generations of star formation, remain unanswered. Observationally, these questions are challenging to address because massive stars are short lived and form in deeply embedded, dusty cocoons, often at large distances. However, their strong ultraviolet radiation ionizes the surrounding neutral hydrogen gas, producing HII regions that emit thermal radio continuum and radio recombination lines (RRLs), providing extinction-free tracers of high-mass star formation (HMSF). Studies of young, compact HII regions therefore, offer insight into the earliest stages of HMSF, while investigations of diffuse and extended HII regions reveal how stellar feedback shapes the ISM and influences subsequent generations of star formation. This thesis approached HMSF from large to small scales, progressively uncovering the properties of ionized and molecular gas. First, a blind search for HII regions was conducted using RRL and continuum data from the GLObal view on STAR formation (GLOSTAR) Galactic Plane survey. This analysis cataloged 244 Galactic HII regions and derived their physical properties and associations with dust and Class II methanol masers (both tracers of young stellar objects). About 48% and 14% of the HII regions, respectively, were spatially coincident with these tracers, identifying a younger, more compact subset of HII regions. Electron temperatures were measured across a large range of Galactocentric distances, allowing the Galactic electron temperature gradient to be derived. Next, the molecular gas associated with the embedded HII regions identified above was studied. A sample of 61 sources was observed in multiple H₂CO transitions with the APEX 12 m sub-mm telescope to probe the interplay between ionized gas, molecular gas, and dust, and to assess how massive stars influenced their natal environments. A non–local thermodynamic equilibrium analysis constrained the kinetic temperatures and H₂ densities, providing a detailed characterization of the dense molecular gas in these clumps. In addition to emission from the main clump, 57% of the sources exhibit multiple H₂CO velocity components, with secondary components exhibiting higher kinetic temperature and broader linewidths, indicative of supersonic, nonthermal motions and turbulent gas. Comparisons between dust, molecular, and ionized gas properties revealed that quantities which are directly linked to the central high-mass star are correlated most strongly, highlighting the star's dominant role in regulating the interaction between its surrounding gas. The third part of this thesis presents a detailed study of the massive star-forming complex, RCW 106, located in the Scutum–Centaurus arm in the southern Galactic plane. S- and L-band continuum data from the MPIfR–MeerKAT Galactic Plane Survey (MMGPS) were used to classify radio sources in this region, including HII regions, supernova remnants, compact sources, and extended emission. Molecular counterparts were investigated using APEX/LAsMA CO observations, revealing a strong spatial correlation with the thermal emission, providing further insight into the feedback processes and triggered star formation. Together, these results attempt to provide a comprehensive view of HMSF, linking the properties of HII regions and their molecular environments across Galactic scales, and revealing how massive stars regulate their surroundings from the earliest stages to fully developed star-forming complexes.