Massive star populations in Wolf-Rayet galaxies

We analyse long-slit spectral observations of 14 Wolf-Rayet (WR) galaxies from the sample of Schaerer, Contini & Pindao. All 14 galaxies show broad WR emission in the blue region of the spectrum, consisting of a blend of NIIIlambda4640,CIIIlambda4650, CIVlambda4658 and HeIIlambda4686 emission lines, which is a spectral characteristic of WN stars. Broad CIVlambda5808 emission, termed the red bump, is detected in nine galaxies and CIIIlambda5996 is detected in six 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 Hbeta emission line, after subtracting the contribution of WR stars. The Schaerer & Vacca models predict that the number of WR stars relative to O stars, N-WR/N-O, 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 indicate an initial mass function (IMF) slope -2 less than or similar to Gamma less than or similar to -2.35 in the low-metallicity regime. For high-metallicity galaxies our observations suggest a Salpeter IMF (Gamma = -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, on NGC 450 as a galaxy with WR characteristics. For NGC 450, we estimate the number of WN and WC stars. The number ratio N-WR/N-O, and the equivalent widths of the blue bump, EWlambda4686, and of the red bump, EWlambda5808, in NGC 450 are also in good agreement with the instantaneous burst model prediction for WR galaxies.