2024ApJS..274...15G

We use the H41α recombination line to create templates of the millimeter free-free emission in the ALMA-IMF continuum maps, which allows us to separate it from dust emission. This method complements spectral-index information and extrapolation from centimeter-wavelength maps. We use the derived maps to estimate the properties of up to 34 H II regions across the ALMA-IMF protoclusters. The hydrogen ionizing photon rate Q₀ and spectral types follow the evolutionary trend proposed by Motte et al. The youngest protoclusters lack detectable ionized gas, followed by protoclusters with increasing numbers of OB stars. The total Q₀ increases from ∼10⁴⁵ s^–1 to >10⁴⁹ s^–1. We used the adjacent He41α line to measure the relative number abundances of helium, finding values consistent with the Galactic interstellar medium, although a few outliers are discussed. A search for sites of maser amplification of the H41α line returned negative results. We looked for possible correlations between the electron densities, emission measures, and Q₀ with H II region size D. The latter is the best correlated, with Q₀ ∝ D^2.49 ± 0.18^. This favors interpretations in which smaller ultracompact H II regions are not necessarily the less dynamically evolved versions of larger ones but rather are ionized by less massive stars. Moderate correlations were found between the dynamical width ΔV_dyn with D and Q₀. ΔV_dyn increases from about 1 to 2 times the ionized-gas sound speed. Finally, an outlier H II region south of W43-MM2 is discussed. We suggest that this source could harbor an embedded stellar or disk wind.