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 , which are likely to be clusters of ultracompact H II regions .
All 35 of the sources presented in this article have positive radio spectral indices ( \alpha > 0 for S _ { \nu } \propto \nu ^ { \alpha } ) , 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 } \gtrsim 10 ^ { 8 } cm ^ { -6 } pc , and energy requirements indicate that the sources in our sample have a range of a few to \sim 560 O7V star equivalents powering their H II regions .
Assuming a Salpeter IMF with lower and upper mass cutoffs of 1 and 100 M _ { \odot } , respectively , this range in N _ { Lyc } corresponds to integrated stellar masses of 0.1–60 \times 10 ^ { 3 } M _ { \odot } .
For roughly half of the sources in our sample , there is no obvious optical counterpart , giving further support for their deeply embedded nature ; for most of the remaining sources the correspondance to an optical source is insecure due 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 } \sim 1.5 \times 10 ^ { 3 } to n _ { e } \sim 1.5 \times 10 ^ { 4 } cm ^ { -3 } and radii of \sim 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 , while 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 HII regions and the analogous massive extragalactic clusters that may evolve into globular clusters .