He II emission as a tracer of ultra-low metallicity and massive star evolution
Résumé
The He II 1640 emission line is arguably the primary probe for detecting ultra-low metallicity star formation in the high redshift Universe but even at z~0 our understanding of it is woefully incomplete. Since the first signs of Population III star formation at high-z is even likely to be seen intermixed with more metal rich star formation, we need a solid understanding of the massive stellar population responsible for the excitation of He II at non-zero metallicity but this is currently lacking. The main reason we do not fully understand the physics behind narrow-line He II emission at low metallicity is that we do not have any direct observational probes of the massive stars responsible for the ionizing radiation. Until we understand this, a significant uncertainty will remain in any interpretation of nebular He II 1640 at high redshift. Here we propose to use COS to carry out the first systematic study of the massive stellar content in galaxies selected to have He II in emission. With this we will a) study in detail the population of massive stars responsible for exciting both narrow and broad He II, b) rigorously test recent claims for significant pockets of ultra-low metallicity star forming gas at 2