OPTICAL SPECTROPHOTOMETRY OF WOLF-RAYET GALAXIES

We have obtained long-slit optical spectra of 10 Wolf-Rayet galaxies and four other starburst galaxies. Using the nebular emission lines we have determined the electron temperatures, electron densities, extinctions, oxygen abundances, mass of ionized hydrogen, and numbers of ionizing photons due to hot stars in these galaxies. The various forbidden line ratios clearly indicate a stellar origin for the emission-line spectrum. From the flux of the broad He II lambda4686 emission feature we have estimated the number of Wolf-Rayet stars present. We have accounted for the contribution of these stars to the total ionizing flux and have calculated the ratio of the number of these stars to the number of O stars. In most of the Wolf-Rayet galaxies the number ratios are an order of magnitude larger than those predicted by current models of stellar evolution under conditions of continuous star formation lasting 10(7) yr or longer. The observed number ratios can be reconciled with the model predictions only if the star formation episodes occur in short bursts with durations of 10(6) yr or less. Wolf-Rayet galaxies are therefore among the youngest examples of the starburst phenomenon, which we observed at a proptitious moment.