Abstract:
Super star cluster (SSC) A1 in NGC 3125 exhibits one of the strongest broad He II λ1640 emission lines in the nearby universe and serves as an important template for interpreting observations of extreme He II emitters at redshifts of z ∼ 2–3. In this talk, I will present results from the Cosmic Origins Spectrograph (COS) observations of A1, demonstrating that the He II line shows no significant contamination from nebular emission in the cluster. I will compare the COS G130M + G160M observations of A1 with recent binary BPASS and single-star Charlot & Bruzual (C&B) simple stellar population (SSP) models, which include very massive stars (VMS) approaching the Eddington limit of up to 300 M⊙. Additionally, I will highlight O V λ1371 line-profile predictions, showing that this line is a critical tracer of youth and the presence of VMS in galaxies.
I will discuss the rarity of He II and O V features, which indicate the presence of short-lived VMS in the galaxy, and I will explore the implications of recent developments in stellar wind theory. Finally, I will emphasize the importance of incorporating VMS into population synthesis models to better understand extreme stellar populations.
