Massive Star Populations in Wolf-Rayet Galaxies

We analyze longslit spectral observations of fourteen Wolf-Rayet galaxies from the sample of Schaerer, Contini & Pindao (1999). All 14 galaxies show broad Wolf-Rayet emission in the blue region of the spectrum, consisting of a blend of NIIIλ4640, CIIIλ4650, CIVλ4658, and HeIIλ4686 emission lines, which is a spectral characteristic of WN stars. Broad CIVλ5808 emission, termed the red bump, is detected in 9 galaxies and CIIIλ5996 is detected in 6 galaxies. These emission features are due to WC stars. We derive the numbers of late WN and early WC stars from the luminosity of the blue and red bumps, respectively. The number of O stars is estimated from the luminosity of the Hβ emission line, after subtracting the contribution of WR stars. The Schaerer & Vacca 1998 (hereafter SV98) models predict that the number of WR stars relative to O stars, NWR/NO, increases with metallicity. For low metallicity galaxies, the results agree with predictions of evolutionary synthesis models for galaxies with a burst of star formation, and indicates an IMF slope -2 <∼ Γ <∼ − 2.35 in the low metallicity regime. For high metallicity galaxies our observations suggest a Salpeter IMF (Γ = −2.35) and an extended short burst. The main possible sources of error are the adopted luminosities for single WCE and WNL stars. We also report, for the first time, NGC 450 as a galaxy with WR characteristics. For NGC 450, we estimate the number of WN and WC