Evolution of Stellar Feedback in HII Regions

Stellar feedback is needed to produce realistic giant molecular clouds (GMCs) and galaxies in simulations, but due to limited numerical resolution, feedback must be implemented using subgrid models. Observational work is an important means to test and anchor these models, but limited studies have assessed the relative dynamical role of multiple feedback modes, particularly at the earliest stages of expansion when HII regions are still deeply embedded. In this paper, we use multiwavelength (radio, infrared, and X-ray) data to measure the pressures associated with direct radiation ($P_{\rm dir}$), dust-processed radiation ($P_{\rm IR}$), photoionization heating ($P_{\rm HII}$), and shock-heating from stellar winds ($P_{\rm X}$) in a sample of 106 young, resolved HII regions with radii $\lesssim$0.5 pc to determine how stellar feedback drives their expansion. We find that the $P_{\rm IR}$ dominates in 84% of the regions and that the median $P_{\rm dir}$ and $P_{\rm HII}$ are smaller than the median $P_{\rm IR}$ by factors of $\approx 6$ and $\approx 9$, respectively. Based on the radial dependences of the pressure terms, we show that HII regions transition from $P_{\rm IR}$-dominated to $P_{\rm HII}$-dominated at radii of $\sim$3 pc. We find a median trapping factor of $f_{\rm trap} \sim$ 8 without any radial dependence for the sample, suggesting this value can be adopted in sub-grid feedback models. Moreover, we show that the total pressure is greater than the gravitational pressure in the majority of our sample, indicating that the feedback is sufficient to expel gas from the regions.