Abstract

We use the H41 alpha 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 Q (0) 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 Q (0) increases from similar to 10(45) s(-1) to >10(49) s(-1). We used the adjacent He41 alpha 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 H41 alpha line returned negative results. We looked for possible correlations between the electron densities, emission measures, and Q( 0 )with H ii region size D. The latter is the best correlated, with Q( 0 )proportional to 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 Delta V- dyn with D and Q (0). Delta V (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.