A sample of clusters of extragalactic ultracompact HII regions

We report on the detection of optically thick free-free radio sources in th e galaxies M33, NGC 253, and NGC 6946 using data in the literature. We inte rpret these sources as being young embedded star birth regions that are lik ely to be clusters of ultracompact H II regions. All 35 of the sources pres ented in this article have positive radio spectral indices (alpha > 0 for S -v proportional to v(alpha)), suggesting an optically thick thermal bremsst rahlung origin from the H II region surrounding the hot stars. The estimate d emission measures for these sources are EM6cm greater than or similar to 10(8) cm(-6) pc, and energy requirements indicate that the sources in our s ample have a range of a few to similar to 560 O7 V star equivalents powerin g 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 c orresponds to integrated stellar masses of 0.1-60 x 10(3) M. For roughly ha lf of the sources in our sample there is no obvious optical counterpart, wh ich gives further support for their deeply embedded nature; for most of the remaining sources, the correspondence to an optical source is insecure owi ng to relative astrometric uncertainty. Their luminosities and radio spectr al energy distributions are consistent with H II regions modeled as spheres of plasma with electron densities from n(e) similar to1.5 x 10(3) to simil ar to1.5 x 10(4) cm(-3) and radii of similar to1-7 pc. Because of the high densities required to Dt the data, we suggest that the less luminous of the se sources are extragalactic ultracompact H II region complexes, those of i ntermediate luminosity are similar to W49 in the Galaxy, and the brightest will be counterparts to 30 Doradus when they emerge from their birth materi al. These objects constitute the lower mass range of extragalactic "ultrade nse H II regions," which we argue are the youngest stages of massive star c luster formation yet observed. The sample presented in this paper is beginn ing to fill in the continuum of objects between small associations of ultra compact H II regions and the analogous massive extra-galactic clusters that may evolve into globular clusters.