The radio properties of NGC 5253 and its unusual H II regions

We present VLA. images of the radio continuum emission of the starburst in NGC 5253 at wavelengths of 6, 3.6, and 2 cm, with resolutions of 1 "-2 ", and at 20 cm with resolution of similar to 9 ". A spectral index map constructed from the 6 and 2 cm images indicates that thermal free-free emission from ionized gas, most of it from a single source, dominates the radio emission. The unusually high fraction of free-free emission suggests that the starburst is very young. Regions of diffuse nonthermal synchrotron emission and probable supernova remnants surround the central radio source. We detect several regions of rising spectral index in NGC 5253, suggesting that the free-free emission is optically thick at 2 cm. These nebulae are situated to the north of the central cluster/main radio source. The turnover frequencies require an emission measure (EM) similar to 10(8)-10(9) cm(-6) pc, and [n(e)(2)](1/2) similar to 10(4)-10(5) cm(-3). From the observed intensities, we infer diameters of a few tenths of a parsec for these regions, which is an order of magnitude larger than the diameters of H II regions of comparable density in the Galaxy. Lyman continuum rates of over N-Lyc similar to 10(51) s(-1) are required to ionize these high densities and volumes. Unlike Galactic compact H II regions, which contain no more than a few O stars, the optically thick H rr. regions in NGC 5253 are excited by an estimated 200-1000 O stars each. This is consistent with the idea that very large clusters are the mode of star formation in the central region of NGC 5253. The optically thick H II regions, located to the north of the dominant optical and radio sources, may well trace the very youngest parts of the starburst.