ar X iv : a st ro - p h / 97 07 27 2 v 1 2 4 Ju l 1 99 7 I ZW 18 – A NEW WOLF - RAYET GALAXY 1

We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad WR bumps at the wavelengths λ4650 and λ5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II λ4686 and C IV λ5808 lines are equal to 17±4 and 5±2, respectively. The WR-to-O stars number ratio is equal to ∼0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the NW component of I Zw 18 can be responsible for the presence of the nebular He II λ4686 emission line, however the observed intensity of this line is several times larger than model predictions and other sources of ionizing radiation at wavelengths shorter than 228Å are necessary. Spectroscopic observations presented herein were obtained with the Multiple Mirror Telescope, a facility operated jointly by the Smithsonian Institution and the University of Arizona. Visiting astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.