NGC 3125-1: THE MOST EXTREME WOLF-RAYET STAR CLUSTER KNOWN IN THE LOCAL UNIVERSE

摘要

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 HeⅡ 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.