2001ApJ...559..864J

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young embedded star birth regions that are likely to be clusters of ultracompact H II regions. All 35 of the sources presented in this article have positive radio spectral indices (α>0 for S_ν∝ν^α), suggesting an optically thick thermal bremsstrahlung origin from the H II region surrounding the hot stars. The estimated emission measures for these sources are EM_6 cm_≳10⁸ cm^–6 pc, and energy requirements indicate that the sources in our sample have a range of a few to ∼560 O7 V star equivalents powering their H II regions. Assuming a Salpeter initial mass function with lower and upper mass cutoffs of 1 and 100 M_☉, respectively, this range in N_Lyc corresponds to integrated stellar masses of 0.1-60x10³ M_☉. For roughly half of the sources in our sample there is no obvious optical counterpart, which gives further support for their deeply embedded nature; for most of the remaining sources, the correspondence to an optical source is insecure owing to relative astrometric uncertainty. Their luminosities and radio spectral energy distributions are consistent with H II regions modeled as spheres of plasma with electron densities from n_e∼1.5x10³ to ∼1.5x10⁴ cm^–3 and radii of ∼1-7 pc. Because of the high densities required to fit the data, we suggest that the less luminous of these sources are extragalactic ultracompact H II region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, and the brightest will be counterparts to 30 Doradus when they emerge from their birth material. These objects constitute the lower mass range of extragalactic ``ultradense H II regions'', which we argue are the youngest stages of massive star cluster formation yet observed. The sample presented in this paper is beginning to fill in the continuum of objects between small associations of ultracompact H II regions and the analogous massive extragalactic clusters that may evolve into globular clusters.