I Z[CLC]w[/CLC] 18: A New Wolf-Rayet Galaxy

We report the discovery of broad Wolf-Rayet emission lines in the MultipleMirror Telescope (MMT) spectrum of the NW component of I Zw 18, thelowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet(W-R) bumps at the wavelengths $\lambda$4650 and $\lambda$5800 are detectedindicating the presence of WN and WC stars. The total numbers of WN and WCstars inferred from the luminosities of the broad He II $\lambda$4686 and C IV$\lambda$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R toO stars number ratio is equal to about 0.02, in satisfactory agreement with thevalue predicted by massive stellar evolution models with enhanced mass lossrates. The WC stars in the northwest component of I Zw 18 can be responsiblefor the presence of the nebular He II $\lambda$4686 emission line, however theobserved intensity of this line is several times larger than model predictions,and other sources of ionizing radiation at wavelengths shorter than 228\AA arenecessary.Comment: Ap.J.Lett., in pres