| Abstract: High-mass stars play a fundamental role in regulating the structure, chemistry, and evolution of the interstellar medium through intense ultraviolet radiation, stellar winds, and supernova explosions. Their ionizing photons create H II regions, making these objects key tracers of high-mass star formation. However, a long-standing problem persists: ionizing photon rates derived from radio observations are often significantly lower than those inferred from infrared data, particularly in compact and ultracompact H II regions. This discrepancy has challenged our understanding of H II region structure, evolution, and feedback for decades.
In this talk, we investigate whether extended radio emission surrounding compact H II regions can account for the missing ionizing flux. We present a multi-wavelength study of eight Galactic H II regions using high-sensitivity radio continuum and recombination line observations from the upgraded Giant Metrewave Radio Telescope, complemented by data from the GLOSTAR survey. Infrared constraints are obtained using Hi-GAL, MIPSGAL, and GLIMPSE data, enabling spectral energy distribution fitting and estimates of ionizing photon rates.
We detect significant extended radio emission around all targets, previously missed by high-resolution observations. Incorporating this extended emission substantially increases the radio-derived ionizing photon rates, bringing them into close agreement with infrared estimates. The ionized gas velocity field is continuous across compact and extended components, indicating a common physical origin. These results reveal a hierarchical structure in compact and ultracompact H II regions, suggesting the leakage of ionizing photons into the surrounding environments.
We also report compelling evidence for a cloud–cloud collision in the ultracompact H II region G18.148−0.283, including velocity gradients, molecular bridging features, and corresponding signatures in ionized gas. Overall, our findings underscore the critical importance of recovering diffuse ionized emission and highlight the transformative potential of future high-sensitivity facilities such as the Square Kilometre Array for understanding massive star feedback and Galactic evolution. |
