A spectroscopic study of a large sample of Wolf-Rayet galaxies

We analyze long-slit spectral observations of 39 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over 2 orders of magnitude, from Z./50 to 2Z.. Nearly all galaxies in our sample show broad WR emission in the blue region of the spectrum (the blue bump) consisting of an unresolved blend of N III lambda 4640, C III lambda 4650, C IV lambda 4658, and He II lambda 4686 emission lines. Broad C IV lambda 5808 emission (the red bump) is detected in 30 galaxies. Additionally, weaker WR emission lines are identified, most often the N III lambda 4512 and Si III lambda 4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. These emission features are characteristic of WN7-WN8 and WN9-WN11 stars, respectively. We derive the numbers of early WC (WCE) and late WN (WNL) stars from the luminosities of the red and blue bumps, and the number of O stars from the luminosity of the HE emission line. Additionally, we propose a new technique for deriving the numbers of WNL stars from the N III lambda 4512 and Si III lambda 4565 emission lines. This technique is potentially more precise than the blue-bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. It is found that the relative number of WR stars N(WR)/N(O + WR) decreases with decreasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue bump EW(lambda 4650) and of the red bump EW(lambda 5808) derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models. We assemble a sample of 30 H II regions with detected He II lambda 4686 nebular emission to analyze the possible connection of this emission with the hard UV radiation of the WR stars. The theoretical predictions satisfactorily reproduce the observed intensities and equivalent widths of the He II lambda 4686 nebular emission line. However, galaxies with nebular He II lambda 4686 emission do not always show WR emission. Therefore, in addition to the ionization of He+ in the H II region by WR stars, other mechanisms for the origin of He II lambda 4686 such as radiative shocks probably need to be invoked.