NGC 3125-1: The Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe^*

We use Space Telescope Imaging Spectrograph long-slit ultraviolet spectroscopy of local starburst galaxies to study the massive star content of a representative sample of super-star clusters, with a primary focus on their Wolf-Rayet (W-R) content as measured from the He II λ1640 emission feature. The goals of this work are threefold. First, we quantify the W-R and O-star content for selected massive young star clusters. These results are compared with similar estimates made from optical spectroscopy available in the literature. We conclude that the He II λ4686 equivalent width is a poor diagnostic measure of the true W-R content. Second, we present the strongest known He II λ1640 emission feature in a local starburst galaxy. This feature is clearly of stellar origin in the massive cluster NGC 3125-1, as it is broadened (~1000 km s^-1). Strong N IV λ1488 and N V λ1720 emission lines commonly found in the spectra of individual W-R stars of WN subtype are also observed in the spectrum of NGC 3125-1. Finally, we create empirical spectral templates to gain a basic understanding of the recently observed strong He II λ1640 feature seen in Lyman break galaxies (LBGs) at redshifts z ~ 3. The UV field observed in local starbursts provides a good overall match to the continuum and weak photospheric features in LBGs in the spectral range λλ1300-1700 but cannot reproduce the He II λ1640 emission seen in the composite LBG sample of Shapley et al. An additional (ad hoc) 10%-15% contribution from "extreme" W-R clusters similar to NGC 3125-1 on top of the field provides a good match to the strength of this feature.