A Sample of Clusters of Extragalactic Ultracompact H ii Regions

Abstract

We report on the detection of optically thick free-free radio sources inthe galaxies M33, NGC 253, and NGC 6946 using data in the literature. Weinterpret these sources as being young embedded star birth regions thatare likely to be clusters of ultracompact H II regions. All 35 of thesources presented in this article have positive radio spectral indices({α}{\gt}0 for S_{nu}~{ν}^{alpha}), suggestingan optically thick thermal bremsstrahlung origin from the H II regionsurrounding the hot stars. The estimated emission measures for thesesources are EM_{6 cm}{\gt}~10^{8} cm^{-6} pc, andenergy requirements indicate that the sources in our sample have a rangeof a few to ~560 O7 V star equivalents powering their H II regions.Assuming a Salpeter initial mass function with lower and upper masscutoffs of 1 and 100 M_{solar}, respectively, this range inN_{Lyc} corresponds to integrated stellar masses of0.1-60{\times}10^{3} M_{solar}. For roughly half of thesources in our sample there is no obvious optical counterpart, whichgives further support for their deeply embedded nature; for most of theremaining sources, the correspondence to an optical source is insecureowing to relative astrometric uncertainty. Their luminosities and radiospectral energy distributions are consistent with H II regions modeledas spheres of plasma with electron densities fromn_{e}~1.5{\times}10^{3} to ~1.5{\times}10^{4}cm^{-3} and radii of ~1-7 pc. Because of the high densitiesrequired to fit the data, we suggest that the less luminous of thesesources are extragalactic ultracompact H II region complexes, those ofintermediate luminosity are similar to W49 in the Galaxy, and thebrightest will be counterparts to 30 Doradus when they emerge from theirbirth material. These objects constitute the lower mass range ofextragalactic ``ultradense H II regions,'' which we argue are theyoungest stages of massive star cluster formation yet observed. Thesample presented in this paper is beginning to fill in the continuum ofobjects between small associations of ultracompact H II regions and theanalogous massive extragalactic clusters that may evolve into globularclusters.