Broad vs narrow He II emission from nearby metal-poor star-forming galaxies

The broad-stellar and narrow-nebular He II λ1640 emission lines are main observational tools to study evolved massive-star populations and their feedback in galaxies near and far. Model galaxy spectra fail to reproduce the observed properties of these lines in local metal-poor star-forming dwarfs. This has negative implications for multiple topical questions, from understanding the properties of the galaxies that contributed to reionization, to understanding the evolution of metal-poor massive stars that give rise to the formation of heavy binary black holes. In an effort to address this issue, we obtained essential observations of He II λ1640 lines in two local dwarfs: NGC 3125 (11.5 Mpc, O/H=solar/3), whose A1 cluster has the most extreme stellar He II line known in the local Universe; and SBS 0335-052 (50 Mpc, O/H=solar/23), which among the nearest most metal-poor starbursts is the most luminous in the integrated nebular He II λ4686 line (the optical counterpart of He II λ1640). In this talk, I present results from the analysis of these two targets, and put the results into the context of observations by colleagues and future observations.