High-resolution radio maps of Wolf-Rayet galaxies: Optically thick HII regions?

We present 20, 6, 3.6, and 2 cm Very Large Array maps and continuum-free Ha images of nine Wolf-Rayet galaxies. The radio maps were made with high spa tial resolution to reject extended emission and to emphasize the star forma tion regions. The ratio of H alpha to radio is at least a factor of 10 lowe r than predicted, indicating that the radio emission is from highly obscure d sources. We map the radio spectral index, alpha (S-v proportional to v(al pha)), across each galaxy. In all the sample galaxies except one, the radio spectrum is much flatter than in spiral starburst galaxies, suggesting tha t free-free emission is more dominant in the dwarfs and that these are very young starbursts that have produced few supernovae. In many of the galaxie s, there are regions where the spectrum I ises from 6 to 2 cm. This require s that the radio emission be optically thick at wavelengths as short as 2 c m. In these optically thick regions, the emission measure, electron density , and ionizing fluxes must be very high, and they are probably the youngest parts of the starburst. The deduced ionization of these sources implies st ellar content of hundreds to many thousands of O stars, which means that th ey may be responsible for a significant fraction of the total infrared lumi nosities of the galaxies. We discuss what these sources imply for the histo ry and evolution of the starburst in each galaxy.