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

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Abstract
We report the discovery of broad Wolf-Rayet (W-R) emission lines in the Multiple Mirror Telescope spectrum of the northwest component of I Zw 18, the lowest metallicity blue compact dwarf galaxy known. Two broad W-R 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 W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass-loss rates. The WC stars in the northwest 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 228 Å are necessary.
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