Extragalactic ultracompact H II regions

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 ({alpha}>0 for S_{nu}_){prop.to}{nu}^{alpha}^), suggesting an optically thick thermal bremsstrahlung origin from the H II region surrounding the hot stars.